The first time my eldest boy (who's just about to turn nine) cried at a movie ( and I mean "broke down sobbing, breaking his little heart" cried) was at the end of Godzilla 2000, when the big G melts down due to his nuclear core. In between full on sobbing and wailing he just kept repeating: " But Godzilla CAN'T die!". I felt both sad for the little fella ,because he was so upset, and simultaneously proud as a Dad - because he was following in my geek footsteps.
To put it in context as well, we'd watched Charlotte's Web the week before and hadn't even blinked (while Dad here got "something in my eye") he had a heart like a rock for that spider. Godzilla dying though, absolutely wrecked him.