Baby Zombie on its own can't go through the trapdoor (covering a 1 block space) as the entity is 1 block high.
At my mob farm I noticed the Baby Zombies riding the Chickens can just go through the trapdoor as the chicken is just 0,7 blocks high. With the trapdoor (0,1875 blocks high) it adds up to 0,8875 so it's just below 1 block high.
Very annoying while playing in survival (almost got killed)

How to reproduce:

• Build a small cage to summon in the baby zombies and leave out 1 block for the trapdoor.
  
  Trapdoor must be vertical.

• /summon minecraft:chicken X Y Z Passengers:[ {id:zombie,IsBaby:1}

  ]}

(For some reason I can't add the { in front of Passengers because when I save it, it says unknown attribute, but that's the command)

• /summon minecraft:zombie X Y Z {IsBaby:1}

(Ofcourse the X Y Z has to be changed to position of the cage)

• Open/Close trapdoor and see the chicken jockey coming through