Baby animals are much smaller and cuter than their adult counterparts. Are the baby piglets, cows, and sheep small enough to fit through a one block hole in a wall?
In previous versions of MineCraft, a baby animal (or villager) had the same hitbox as a parent (even though they looked smaller). But, in version 1.5 this was fixed so babies are smaller than their parents.
As has been pointed out, pigs and chickens are already only 1 block tall, so an even smaller baby animal makes no difference.
However, adult cows, sheep, and villagers are taller than one block, allowing them to be filtered from the babies who can fit through a 1 block gap.
Upon further research, this attribute appears to be the same as the parent.
Pigs, both big and small, can.
Adult sheep cannot.
Little sheep also cannot. Try as I might, I could not get this little guy to pass under, although he looks short enough.
Cows cannot. I don't think I need to test the adult cows, but I got the same story with the calf as with the baby sheep.
I wasn't fast enough to screencap the chickens, so just trust me when I say, chickens are definitely small enough.
I have come to the conclusion that sheep definitely can. So all babies should be able too.