My DH is a childrens football coach on a Saturday morning. I think his club has a good balance between competitiveness and fun. They have a squad of about 14 players, 12 of those will be regularly picked for league games. The league games are competitive, and DH will usually pick the strongest team he can. There are a couple of children who unfortunately will never be picked as their ability is way behind the others. However, these children are always first choice for any friendlies, and all the children attend training together once or twice a week.
Our two sons both play for DHs team, and they would always be picked. While I know that this may be unfair to some of the other children, my DH puts in hours of time each week, not just attending matches, but also training sessions, liaising with other teams, going on first aid and training courses, paperwork, chasing subs, talking to parents about their child's progress. All this is totally voluntary, and most parents do understand that our sons will be picked first. This seems reasonable to me, it would be unreasonable to expect DH to look after 12 other children ahead of his own.
Parents can be a bit of a problem , sometimes my DH says that it seems like there are 10 coaches on the sidelines, all shouting different instructions, the kids get very confused! This has improved though since DH had a quiet word pointing out how confusing this is for the kids, they don't know which voice to listen too!
Of course, there will always be coaches who's style a parent may not agree with. If you genuinely feel there is a problem, talk to the clubs child welfare officer. How do the other parents feel about this coach? If he's not your child's coach, probably you should leave it to parents who's children are in this coach's team to resolve, but you could advise them how to proceed with any complaints they have.
But ultimately , please remember that all coaches are volunteers, giving hours of their own time each week for your child. You may not agree with how things are done, doesn't necessarily mean that they are wrong. The OP gives no details of the actual problem, but the child welfare officer would be able to advise.