I am relatively newly qualified. I recently had a U9 match. I had nothing to do, apart from one free kick, which was obvious. Now I'm not saying let everything go, but they will be there to enjoy playing football, not like they are trying to win the Champions League. Try and coach them through the game, teach them, but if you have to, then do use cards, as a referee you have to follow the Laws and enforce them. But, as I said, you shouldn't have much to worry about. For the U12, I also had one of them, but my only problem was that the U12 followed straight after the U12! So, I was to lenient, and let the game flow too much, but that was only because I had so little to do in the U9! I realise I should've toughened up for the U12, and so the next match I had was an Under 12. To calm the match down, the first foul I gave was in the first 30 second, and it was a border line foul, but it let the players know I was there, I would give free kicks, and that they couldn't get away with it. It set out what I was going to do for the match, and that way, there was no arguing for any other fouls I gave. So, my advice for the U12, is to give a free kick that is a border line free kick. That way, the match calms down, and the players know you are there to mean business. Also, just have another look over the laws regarding the penalties to decide a match, because you want to make sure that you know how the correct procedure goes on, so if it comes to them, you know exactly how it works, I had a cup game at weekend, and that went to penalties. Good luck for the matches.