Hmmm.... I have three common answers, depending on the "angle" of the parent:
1)" My son would have more fun playing QB" (or RB or Middle Linebacker -whatever):
"Every boy would have more fun playing QB... but I don't have any formations in my playbook with 11 QB's on the field. So my son plays Guard, and yours plays Center because I'm sure they both wants to have the most success for the team they can, and feel like they contribute their best skills to the position that let's them do that. This is why I have my son at Guard, and yours at Center."
2) "My son would be better at RB than the other boy":
I also address the MPR rule when a parent says "Couldn't you just switch my little Bobby with Tommy so Bobby can run the ball"? I explain "Well, then Tommy will have to move to his next natual position, which is currently Johhny's spot, so then Johnny has to move to his next spot, which now belongs to Sammy.... so me moving your one boy means half the team get's moved around to new positions... so tell you what, you devise an entire roster showing where every boy should play, just to get your son the ball. If all the coaches like your linup better than ours, and all the other parents feel moving your son is important enough to make their boys move out of their position, then we'll do it!"....
3)" I don't agree with your play calling":
The key thing I let the parents know is "I have done this for 12 years, and volunteered thousands of hours to my coaching. If you really have this much interest in player evaluation, training, scheme development, and play calling, you should consider signing up as a coach next year to try it out... but for now, we are going to have to move forward playing the best jersey at the most likely position"
Coach Magee