Minecraft wiki entry for Zombie, associated text: They are much faster than the normal zombies, yet they have the same health.
The noises they make are higher-pitched than adult zombie sound effects.
If a baby zombie is riding a mob that is also rideable by a player, that mob becomes un-rideable by any player.‌[BE only]
Baby zombie villagers turn into baby villagers if cured.
Worn armor shrinks to fit their body size.
They are able to fit through 1×1 block gaps.
They give 12 experience points when killed by the player or a tamed wolf instead of 5.
Unlike most other baby mobs, they remain babies indefinitely and never become adult zombies.
They have a decreased hitbox size.
A baby zombie has a 15% chance to become a jockey when it tries attacking.‌[BE only]