You can leave some of these things to U16-18 players, but younger players mostly can't do these things. Now the parents play an important role, if you don't want to do it all yourself. You'll profit from it during your training. You need to know what you'll do with this training, so you have to make plans. For example, in matches of U6-10 you continuously use different substitutes.
Some coaches let their best players play longer than less skilled players.
Some parents even agree with this, because the team gets stronger by it. But when you give every player the same substitute minutes, and you make this clear to the parents, you may keep yourself out of much trouble. Also, the club and the coach in particular have to be honest and clear about 'selecting' youth players, because this is often a topic of discussion. Make clear how the selection procedures work. I heard of a coach who felt she had difficulty outvoting the parents during the matches.