Baby villager hitbox changes after their beds are destroyed or after waking them up. You cannot hit them in the heads as you can break/place blocks behind the villager child's head. The hitbox is the torso of the baby villager meaning that it is the only place where the hitbox is placed. They also look different when they are staring at the player. The reason why this happening is because the baby villager's hitbox is using the sleeping hitbox (when villagers return home and go to sleep on their beds.).

To Reproduce
1. Time set night/midnight
2. Spawn baby villager (with villager spawn egg)
3. Place bed, wait for the baby villager to sleep, and destroy the bed or wake villager

Once you've reproduced this, point your crosshair at the baby villager's head and try to punch them in the head. You will notice that they don't get hurt. You can also use a fishing rod to fish the baby villager, you can see where the bobber is placed.