If the issue is overcrowding, control the NUMBER of patrons.
Controlling the makeup of the lounge population is simply a way to make Platinum customers in the now excluded class feel that Amex doesn't think they're paying a high enough annual fee.
In this case, it's families traveling together. Show up at an off hour to a half empty lounge and be asked to pay 50 bucks to get the second kid in?
Yeah, that's not an attempt at monetizing family travelers. That's trying to control overcrowding. Sure.
Btw, I have no minor children and travel almost exclusively solo or as a couple.