I imagine that this can be a tough predicament for parents, often feeling frustrated and powerless in being unable to help their child. Unfortunately, there isn't a whole lot you can say unless you are the coach him/herself. Many coaches already have their own coaching style instilled in them and can be difficult to sway without good reason. Could you imagine if every parent of every child approached the coach, asking him to play his/her child more, and the coach complies? The team would lack any kind of fluidity, structure, and would act more as a Little League team than a high school one. Surely the coach would lose his/her job almost immediately.
BUT, in no way does this mean that your child has to sit their helplessly and keep the bench nice and toasty for the star players. Many coaches appreciate a strong work ethic, dedication, and leadership. Your child may not be the most gifted athlete but your child can very well be the hardest working. I can suggest your child talk to the coach and express eagerness to improve his/her weaknesses and desire to help the team the best way possible. He/she should jot down notes on what the coach would like your child to work on or improve. Have your child lead by example...be the first one in at practice and the last one out. A student of the game and someone who works extremely hard rarely goes unnoticed. Moreover, maybe the extra work and added dedication will be just what your child needs to breakthrough that hump and achieve significant playing time. Your child's actions will influence a coaches decision much more and speak much louder than any combination of words you put together.
I'm not sure how big a football fan you are, but I want to leave you with this quote that I think will resonate well with you and your child:
“I was never the biggest, I was never the fastest, and I sure as heck was never the strongest. The only bread and butter I had was work. At the end of the day, it's all about hard work. And if you outwork me, you're one heck of a man." - Ray Lewis, future Hall of Fame linebacker
I wish you and your child the best of luck! Go get em!