with a certain position then I'd have him coach it regardless of whether his kid played it or not.
I actually coached the defense when my son was starting out, I didn't become the head coach until he hit 4th grade and by that time he was well established as the best qb in our league so no one really questioned it. But even as the HC I still ran the defense, Chase Roberts dad Kyle ran our offense from 4th grade on until they went to HS.
But regardless of how successful you are there's always gonna be someone that questions you.