If a Baby Zombie (and possibly baby Zombie Pigmen as well, but I didn't test it) picks up anything from the ground, killing it won't make it drop the item, whereas an Adult Zombie would.

Way to reproduce:
1. Go to Creative and keep spawning zombies until you get a baby.
2. Give it anything, and see if it picks it up.
3. If it does, kill it and you'll see it doesn't drop it.

(Could this be because they're coded not to drop anything at all?