If you make a strong first impression, that training should be taken as seriously as school work, and you clamp down hard on any unnecessary silliness then they'll usually get on just fine. Most of them are just happy to play football, if the kids are being silly etc it means they're bored but since I let them create their own game, they got on just fine, cause they knew what they liked.
Perhaps I did inherit a more mature group than most but from my experience, there were even more mature groups than mine around so I think you comment includes a rather unfair generalisation. Most of the boys really enjoyed having that responsibility and grew with the task and both the kids and the parents thanked me after the season for not treating the kids like "babies" and treating them like adults.
(I was the main coach btw)
I wouldn't take note of the negative comments.
My first go into kids coaching has seen a group of 9 year olds wanting to do a lot themselves, they can get into drills themselves, play some of the games and ask to do particular drills they enjoy.