Short Summary:

Baby zombies and baby husks aren't damaged (or only sometimes are damaged) by standing on magma blocks.

What is expected:

Baby zombies and baby husks should be damaged by standing on magma blocks like other mobs.

Longer Description:

In a survival world, I have a mob farm where the mobs fall to their death. The babies didn't always die, so I placed a layer of magma blocks down. I later saw babies just standing on the blocks without being hurt.

I recreated this in a creative world. In the creative world, sometimes the baby zombies would be damaged by standing on the block and sometimes they wouldn't. It seemed to depend on exactly where they were positioned.

It also may depend on whether the baby was riding on an adult previously?