In that time the belief was that Jesus was perfect already when he was born, thus he was painted as an adult. This spread to other religious paintings containing children too, thus we have these creepy kids.

You would look weird too if you had been through what they had!

They weren't children after all; they were adults and the pickling shrunk them

I believe they're called homunculus. I couldn't find too much on them, but here's an article. https://www.thecollector.com/baby-jesus-in-medieval-religious-iconography

They were brined.

That is what they looked like in the olden days.

You see, infant mortality was very high, because instead of practicing good hygiene, people practiced pickling babies, and so only the most ripped and strong kids made it to 5.