Baby chickens will not swim up when under water and drown. Baby pigs do swim up but do not get high enough out of the water and drown. I haven't tested all baby mobs, but I have tested baby sheep, baby pigs, baby horses, baby cows and baby chickens.
MC-134635
Some baby mobs having swimming issues
Resolved
Duplicate
1
William Chapple
(Unassigned)
Unconfirmed
Minecraft 1.13
Retrieved
You are absolutely correct! I went ahead and tested some more for you (in Creative)
Ocelot Kittens do not float and drown
Wolf puppies float and drown
All other passive mobs seem to float and not drown.