Perhaps those servers that are actively encouraging a child to order from the adult menu are doing it as they want a larger tip. They don't care about the credits as they are pooled and they can get away with it. Perhaps the servers are bending the rules to line their own pockets.
I wouldn't go into having the DXDP expecting to be able to do this. I would say if you know your child wants to order from the adult menu then you should probably up their age. I guess if it gets offered or even encouraged then maybe it is a bonus to your child, but yes you should then be tipping the server more.