How about allowing kids in settlements and some basketball nets they will play with to increase happiness? Perhaps even some in-game mini-games with other settlers.

And some side quests that allows selling those kids to experiment section for some nice stuff? As long as doing so doesn't alert the other settlers ...

Settlement infirmary for older settlers to increase happiness.

There is one kid, perhaps 12 years old, in one of my settlements. I can't order him to do anything, he just stands around complaining about being bored.

I bet you can't order the kid around just in case you wanted to make them do something dangerous or some other crap like that. Like having the "essential" flag wasn't enough.

Settlement should have fall pits to catch enemies alive, who can then be put in cages, so the children can watch them and increase happiness, and some can be placed in a fighting contest for excitement and the rest sold for slave labor to increase income.