If you sent them out of the house after properly training them, they net you some influence and other gains daily without taking up space in your house. My kids are not fully trained yet but they can provide some neat bonuses if I send them off.
Also, flavor reasons. For example, I wanted to make a goon squad made up of me, my 3 sons, and Ayden. (That did not pan out because their Toughness did not go up through the Age+1 menu despite the damn wiki saying aging a kid raises stats. There's code in the files but it does not work.)
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