Originally Posted by
Zero
Well, some still bear the Senju name, like Tsunade (IIRC), so was her little brother (Tsunade must have been an extremely early child seeing how great the age difference is between them); some are dead (like those that have been experimented by Oro, to give them Hashirama's power, because I doubt that he picked children at random), Tenzou might be connected to the Senju clan, seeing as he was able to adapt to Hashirama's power as a child; if we believe Tobi, some of them are called Sarutobi and some are Namikaze and the like (this is mainly suggested)... in short, it looks like the Senju surname was simply lost. For example, they might have died before marrying their lovers which were already pregnant; most Senju had daughters which later took the names of their husbands, or even those that changed their names to not be target for their surname only, or just giving their children surnames of their mothers for the same reason, just like in Naruto's case (he was given his mother's surname so that he won't be targeted by Minato's enemies).