By right, this trope should be pretty clear - it's about an adult character who often behaves childishly because
they interact well with children. It's different from Manchild
because, unlike the latter, the former is capable of conversing/behaving like a normal adult. It's only when younger characters are involved that they behave like Manchildren
However, as someone pointed out in the IP thread, most of the examples listed on this page are simply "people who behaves childishly".