Minecraft wiki entry for Zombie_Villager, associated text: They are 30% faster than the normal zombies, yet they have the same health as normal zombies. This makes the baby zombie villagers more dangerous than their bigger counterparts.
The noises they make are higher-pitched than adult zombie villager sound effects.
They can sometimes ride chickens. They can also ride‌[Bedrock Edition only]:
Cows
Ocelots
Wolves
Mooshroom
Adult chickens
Pigs
Sheep
Horses
Donkeys
Mules
Zombie horses (unused jockey variant)
Skeleton Horses
Adult zombies
Adult zombie villagers
Pandas
Adult husks
Adult zombified piglins
Stray cats
Spiders
Cave spiders
It moves at the baby zombie villager's speed, instead of the mob's speed.
If riding a neutral mob, it becomes hostile, but does not anger any mobs nearby that are the same mob as they are.
If a baby zombie is riding a mob that is also rideable by a player, that mob becomes non-rideable by a player.
A baby zombie has a 15% chance to become a jockey when try attacking villagers, players or golems.
Baby zombie villagers turn into baby villagers, if cured. In Java Edition, their heads turn back to small baby villager heads.
Worn armor shrinks to fit their body size.
They are able to fit through 1×1 block gaps.
They drop 12 experience when killed by the player instead of 5.
Unlike most other baby mobs in the game, they stay as babies indefinitely and never grow into "adult" zombie villagers.
They have a decreased hitbox size.