Unfortunately dealing with parents is part of the job, the thing I use to do at the beginning of the season is to hand out a letter to every parent outlining what I plan for the season. Things I specifically mention are that I don't care about winning, scoring, positions, what I want is for the kids to have fun, maybe learn a little bit about soccer and want to play again next year. All kids WILL get equal playing time regardless of ability. At that age, technically our kids teams all are "random" however alot of parents will write in requests for certain coaches, either to have them or not, or that they want to be little Jimmy's team or some such. We also keep all family members together to ease driving issues with the parents. What are the issues the parents are nagging about? If we know that maybe we can offer some suggestions. You do need to listen to the parents, occassionally they will notice something you miss.
For what it is worth, if you're still reading this thread, I coach a U-8 team with knowledgable soccer parents - many coaches themselves and former professionals - and for the most part, every kid plays every position. That's the only way they'll learn the tools they need. For example, if you play a kid at sweeper, he will not learn how to play with people behind him. If you play a kid exclusively at forward, he won't know how to defend a one-on-one. Kids learn very fast at this age and develop habits, so it is best to teach them how to play the game first, and to do that they must play every position. (my keeper only comes out of the net at the end of games when we're ahead, I think a keeper is a keeper and must develop as such). The reality is that both parents and kids have the most fun when they win, so at the beginning of the game, to get a lead, I will have the players at their most effective positions, we'll score goals, then I can move them around. Even if we're not ahead I move them around. If the game is close, I will simply make sure at the end of the game to put the kids back in their most effective spots. As for stretching and warm-up, their muscles don't need it. The most important part about the warm-up is getting their heads in the game and getting them focused on soccer. Game-like drills, two-on-ones to the goal, is a great way to get them prepared so that the transition from warm-up to game is seamless. As for the annoying parents, that's always a pain. It is best to just close practice to parents and only communicate about soccer through email. At least that's how I cope.
Our U8 kids play 4 a side on a small field (including the keeper). U6 kids play 3 a side on an even smaller field. I don't worry about positions at all. Other than 1 kid back to play in the net as required, they have total freedom. I only tell them to win balls off the other team and not to give the ball away when they have it. They seem to enjoy it that way. I also try to get my 'keeper' to use his feet more often than not.