I wouldn't really recommend pay leagues. For a simple point that does feed into the competitive nature of said leagues. If I were in a pay league and had a father with two young sons join it I would feel like it was in the end three teams being controlled by one manager, and since most pay leagues are leagues full of strangers (not met in person) league integrity is vital to maintaining the league.
There is also the fact that if you were in a pay league would you want it to become in essence a training league.
I also have no idea what pay league you are in Ultrasurblanco, but the one I run has more activity in it then my other five leagues combined.
I still feel you need a league where you control the majority of its aspects. In a lot of leagues if you come in and start asking for things to be done a certain way, or for certain other mangers to get kick out because of behavior it could result in you being removed.
For instance if you come to be and demand that someone that I've played in leagues with for five or more years be removed from the league for constant trash talk, I'll likely side with the manager whom I know is going to stick with their team if given a ultimatum.
I also recommend that you and other adults in this league just be co-managers. I doubt that you'd want any of the adults taking advantage of the kids in trades, and it would be difficult to tell the kids that they couldn't trade with a adult team whenever said team had their favorite player. Consider it like coaches pitch little league for fantasy baseball.