Even considering how Littlefoot lost his mother, to me it would seem out of character for his otherwise sweet nature to want to outright kill a newly-hatched Sharptooth, since Chomper was only a cute baby and hadn’t done anything yet.
That said, it also seemed odd to me that of the five kids, Littlefoot was the most insistent on caring for Chomper. If anything, it seems like Littlefoot should’ve been the most apprehensive about it. I think there was potential for some interesting character development there.