Mikeitz, Bereishis 42:37. Reuven's Guarantee and the Relationship with Grandchildren.

Shimon was imprisoned in Mitzrayim, and the brothers told Yaakov that they had been warned not to return to Egypt without Binyamin.  When they found their money in their sacks, Yaakov was distraught, convinced that if he sent Binyamin he would never see him again, and that Shimon was finished.  Reuven told Yaakov that he guaranteed the return of Binyamin; he said  את שני בני תמית אם לא אביאנו אליך תנה אתו על ידי ואני אשיבנו אליך, he was so sure that he could bring Binyamin back that he told Yaakov that he could kill his two sons if he didn't bring Binyamin back.  Rashi brings the Medrash that לא קבל דבריו של ראובן. אמר בכור שוטה הוא זה, הוא אומר להמית בניו, וכי בניו הם ולא בני.  Yaakov rejected Reuven's words.  He said "This is a deranged firstborn.  Are they his children and not mine?"

The Maharitz Chiyos in his Toras Hanevi'im (in the note there) wonders how Reuven could seriously make such an offer.  Is one's child's life a commodity?  He suggests that perhaps Reuven's ability to make such an offer was pursuant to his status as leader of his brothers and presumptive king of the nascent Klal Yisrael.

The Chasam Sofer in his last teshuva in Orach Chaim, written to the Maharitz Chiyos, disagrees, and says that this was just Reuven's way of saying that he would give up his din bechor, his claim to a double inheritance.  "שני בני" means his bechor status, which gave him a claim as if he were two heirs.  He explains that the expression "תמית" is just another way of saying that he would give up his claim to land in Eretz Yisrael.  The Gemara in Bava Basra 118b says that owning land in Eretz Yisrael is referred to as חיות, life:

 מרגלים יהושע וכלב נטלו חלקם.  מנהני מילי? אמר עולא דאמר קרא (במדבר יד) ויהושע בן נון וכלב בן יפנה חיו מן האנשים ההם; 

and the Gemara says מאי חיו שחיו בחלקם, so you see that חיות can be a metonym for ownership of land in Eretz Yisrael.  Reuven was saying, if I fail, you can take away my bechora-rights, which would mean cutting his share in Eretz Yisrael in half.  The Chasam Sofer adds that according to his pshat, the Medrash should not be read as if Yaakov was ridiculing Reuven's attitude that his sons were his and not Yaakovs, but rather that Yaakov was saying that he had already taken away the bechora from Reuven- "Do you think your family is entitled to a double portion as if your two sons were my two sons?  They're not.  I've already decided to strip you of your bechora.  They're your sons, not mine, and you're entitled to no more than a regular heir."

Rav Baruch Epstein says this pshat also, and he makes it clear that his way of putting it is superior to the Chasam Sofer's.  

אפשר לפרש הענין [ע״ד הדרש] על פי מ״ד בב״ב , ויהושע וכלב חיו מן האנשיס  התרים מן הארץ, ופריך בגמרא , מאי חיו, אילימא  חיו ממש והכתיב קרא אחרינא (פ׳ פינחס) ולא נותר  מהם איש כי אם כלב בן יפונה ויהושע בן נון, אלא מאי חיו — שחיו מחלקם, כלומר שנטלו חלקם של  מרגלים בחלוקת הארץ [ע״ד מה שאמרו זכה — נוטל  חלקו וחלק חבירו], ויהי׳ לפי״ז באור הלשון חיו  כלפי שאמרו עני חשוב כמת נקרא העשיר חי   ולפי״ז כשנרצה לומר פלוני נתעשר מפלוני אומרים  פלוני נחיה מפלוני, ומבואר לפי״ז דכמו שהנוטל  חלק בנחלה נקרא חי,  כן המאבד חלקו נקרא מת וע"ד זה אפשר לפרש כוונת דרשת חז"ל עה"פ דפרשת משפטים וגם בעליו יומת דקאי על עונש ממון, כנודע.  והנה ידוע דיעקב הנחיל לבניו ולבני בניו את ארץ כנען, כמבואר בפ' ויחי בברכתו לאפרים ומנשה  ובמדרשים ואגדות, ועל זה אמר ראובן, אשר אם  לא ימלא משלחתו בבנימין להשיבו אז ירשה לו שיאבד  בצואתו זכות נחלת שני בניו בארץ כנען, והוציא ענין זה בלשון תמית ע״ד המליצה כלפי אלה הנוחלים  שנקראו חיים כמו שבארנו, וע'ע בפרשה הסמוכה  בפסוק ע׳ מש״כ שס. [לאחר שנים רבות שכתבתי  זה הייתי בע״מ וויען  והגיד לי שם אחד מחכמי עיר  פרעסבורג כי הגאון חת״ס בכתביו נגע ברעיון זה]++ זה.

If Harav Epstein had picked up the Teshuvos, he would have seen that the Chasam Sofer was indeed נוגע ברעיון זה, and that his version is not substantively superior.  One can, however, enhance the Chasam Sofer by reference to the Brisker Rov in Vayechi (48:16) as follows.

In Parshas Vayechi, Yaakov gives a bracha to the sons of Yosef.  In 48:4, it says ונתתי את הארץ הזאת לזרעך אחריך אחזת עולם.  Then, in 48:5 it says שני בניך הנולדים לך בארץ מצרים עד באי אליך מצרימה לי הם אפרים ומנשה כראובן ושמעון יהיו לי.   Finally, in 48:16 it says ויקרא בהם שמי.  We immediately realize that these two sons of Yosef have thus supplanted the sons of Reuven- they were counted as separate shevatim, thus granting a double portion to Yosef.  Now, the Rov brings from the Rashbam and the Ramban that ויקרא בהם שמי means שיחיה זרעם וזרע זרעם, or שיעמוד זרעם ושמם.  He explains that passuk 4 shows that the essence of the Bracha was inheritance of the land; 5 shows that this bracha gave Yosef's sons the status of being Yaakov's sons and thereby independent Shevatim, who are therefore entitled to individual portions, thus yielding for Yosef a double portion.  The Gemara in Bava Basra 115b states אמר אביי גמירי דלא כלה שבטא, we have a kabbala that no Shevet will be entirely wiped out.  Also, he brings that the Toras Kohanim Bechukosai 8 says ברית כרותה לשבטים, and the Raavad there says ברא הקב"ה ברית שלא יכלה זרעם.  According to this, we can see how meduyak the Chasam Sofer's idea is in the words of Reuven and Yaakov:  If Reuven's sons would have the status of Shevatim, this would yield two results: double portion of land, and a guarantee that they and their names would survive forever.  By waiving his claim to the Bechorah, Reuven exposed his children to the possibility of death.  We don't need the Chasam Sofer's connection of the word חיו to ownership of land.  The חיות at issue was real חיות, the guarantee of survival, and that is why Reuven said את שני בני תמית.

The Aruch LaNer, in his Binyan Tziyon, says he doesn't like the Chasam Sofer's pshat, (although I don't think his complaints are very strong,) and suggests a different pshat.  He says that Reuven was making an oath that he would bring Binyamin back.  The Gemara (e.g., Shabbos 32b,) says that transgression of an oath can cause the death of minor children.  At that time, two of Reuven's four children were minors, so he was telling Yaakov that he was willing to make a Shvu'ah that he would bring Binyamin back, even knowing that failure to fulfill the oath would be fatal to his two minor sons.   With a little imagination, the Aruch LaNer's pshat can be read into the Targum Yonasan, who says ית תרין בניי תקטול בשמתא אין לא איתיניה לוותך.

What I find most interesting here is the apparent difference between the perspectives of Reuven and Yaakov about the relationship between a person and his grandchildren.  Reuven held that the connection of Yaakov to Reuven's children is contingent on Reuven.  If Reuven's relationship with Yaakov would be destroyed, then his children would no longer have anything to do with Yaakov.  Yaakov held that Bnei Banim K'Banim, that the relationship to a grandchild stands independent of the relationship to the child.  Obviously it stems from that first connection, but it acquires independent significance.  Even if a child were disinherited and estranged, the relationship to the grandchildren would not necessarily be affected.  It could be that this is the Shakla v'Tarya of the Gemara in Yevamos 62b, where it's pashut that בני בנים הרי הן כבנים as far as the son of a son, but it's a chidush that it applies to the son of a daughter.  It also comes up in the din of yerusha b'kever in Yeish Nochlin in Bava Basra 116b, as follows:

בעי רמי בר חמא אבי האב ואחיו כגון אברהם ויעקב בנכסי עשו איזה מהן קודם אמר רבא תא שמע האב קודם לכל יוצאי יריכו ורמי בר חמא יוצאי יריכו שלו ולא יוצאי יריכו של בנו ה"נ מסתברא דקתני זה הכלל כל הקודם בנחלה יוצאי יריכו קודמין ואילו איתיה ליצחק יצחק קודם השתא נמי דליתיה יצחק יעקב קודם שמע מינה:

Rashbam there:

אבי האב. של מת והאחין של מת דהיינו זקנו ואחין איזה מהן קודם מי לימא כיון דיצחק קודם ליעקב לירש את עשו דהאב קודם לאח הרי הוא כאילו ירשו יצחק לעשו והלכך יבוא יעקב ויירש את יצחק אביו אבל אברהם לא יירשנו שהבן של מת קודם לאבי המת או דלמא בני בנים הרי הן כבנים ונמצא עשו בן אברהם וקודם אברהם ליעקב בנכסי עשו דהאב קודם לאחין: 

So it turns out that this question is at the bottom of Rami bar Chama's iboya.  

The question also arises in matters of family law- in many, if not all states, grandparents have an independent right of visitation with their grandchildren, except where the exercise of such rights infringes on the superior rights of the parents.  This should be tolui on the machlokes between Yaakov and Reuven, the Gemara in Yevamos, and the iboya of Rami bar Chama in Bava Basra.