The curse of a Chacham always comes true (Makos 11a). When did that of Reuven materialize?
Ba'al ha'Turim: It materialized with the death of Dasan and Aviram in Parshas Korach. 1
Ohr ha'Chayim: "Leimor" means that he said words implying that his sons would die, but he was careful not to explicitly say it like a curse, therefore it was not fulfilled.
See Ba'al ha'Turim. (Sechel Tov 35:16 implies that also the curse of a Tzadik is always fulfilled.)
What is the meaning of "Tamis"?
Seforno: He meant that Yaakov should curse them that they should die. 1
Riva #1, Hadar Zekenim #1: If I do not return Binyamin, I am liable that my sons die due to you, i.e. due to my sin against you.
Riva #2, Hadar Zekenim #2: You should make them poor, and an Oni is like a Mes; you will not give to them an inheritance in Eretz Yisrael. 2
Targum Yonasan: What Reuven meant was that Yaakov should place on them a Shamta - a form of excommunication. 3
Oznayim la'Torah: Reuven was accepting upon himself a Neder to return Yosef, and the punishment for not keeping a Neder is that one's minor children die. 4
Ohr ha'Chayim: It is to punish.
Malbim: The verb is third person feminine - the sin will kill them. You fear punishment lest he be harmed. I accept it on myself!
The Seforno adds that he wanted him to curse them already now, and that should he fail in his mission the curse would take effect retroactively - like Rav cursed Shmuel, in Shabbos 108a.
Hadar Zekenim: This is why it says "Yechi Reuven v'Al Yamos" (Devarim 33:6) - The inheritance of his sons is greater than others, like we find in Sefer Yehoshua. (The Ramban (to Bamidbar 35:14) says that Ever ha'Yarden was very big. There were more men in Shevet Reuven than in Gad or in half of Menasheh, so it is reasonable that Reuven's share would be biggest. I do not see a source from Sefer Yehoshua (13:15-21) which lists cities of Reuven, but far more cities of Yehudah (Yehoshua 15:21-62), and in Ever ha'Yarden itself, Menasheh received all 60 cities of Og (Yehoshua 13:30)!) (PF).
Perush Yonasan: As was common in those times. See for example Shabbos 130b and Bava Metzia 70a). See also Na'ar Yonasan.
Oznayim la'Torah: This also explains why Reuven said "Shnei Banai" - since two of his sons were minors. (Why did he say "Tamis," i.e. you will kill them? Perhaps this is third person feminine, like Malbim says; the sin will kill them. - PF)
Reuven had four sons (Chanoch, Palu, Chetzron and Karmi). Why did he say "my two sons"?
Ramban: What he meant was that in place of not returning one of Yaakov's sons, two of Reuven's sons would be punished. 1 Ohr ha'Chayim - He kept two sons, 2 lest he lose something in the world to come 3 (i.e. the Mitzvah of Peru u'Revu). Yaakov did not accept this, therefore Yehudah offered to endanger also his Olam ha'Ba if he will not return Binyamin.
Oznayim la'Torah: Refer to 42:37:2:5*.
See Oznayim la'Torah, who rejects this explanation (without quoting the Ramban).
Ohr Yakar: This need not be like Beis Shamai. This is even like Beis Hillel, according to R. Bun (Yerushalmi Yevamos 6:6, who says that Beis Hillel hold that one is Yotzei through a male and a female, or two males).
Even if Reuven retained Peru u'Revu, would he not lack in the world to come for causing the death of two sons?! Perhaps he means that his body should not lack; if one neglected a Mitzvah, the corresponding limb will be maimed in the world to come (Mishlei 13:13, according to the Gra, Shemiras ha'Lashon, introduction, DH uvi'Frat. - PF)
Why did specifically Reuven make this offer?
Ohr ha'Chayim: He was not liable for selling Yosef, so he was sure that he can return Binyamin. Ha'amek Davar - When he was with the brothers, he was punished with them (Bava Kama 92a). If the burden is only on him, he will not be harmed. 1
Ha'amek Davar: Yaakov suspected the brothers of liability for Yosef and Shimon; presumably, Reuven the oldest, had a share in this. Reuven wanted to convince him otherwise by accepting responsibility for Binyamin. 2
