I've discovered this recently and wondered why those boards are in a separate category.
I suppose it's because the AARoads Shield Gallery is a subset of the "Photos, Videos, and More" sub-forum. Or how Sports is a part of the Off-Topic forum (the "parent" forum to it).
I dunno, computing terminology likes that kid of hierarchical structure.
'child board' is a term of the forum software itself. it has nothing to do with biological offspring, and is indeed a term to represent hierarchy. a sub-board is the 'child' of a 'parent' board.
It's because you can't touch them until they turn 18.
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi1.kym-cdn.com%2Fentries%2Ficons%2Foriginal%2F000%2F000%2F005%2Fpedobear.jpg&hash=08b94085d9dd5c1fc06d24d9888979bbd144ec2b)