And also
@HullRef no-one is saying or has said to send a kid off. My opinion and approach is that you can't be afraid to use your cards because of their age. At such a young age, they are getting to know the laws. I used to coach kids and respecting the permitted distance is a day one, week one drill. They know better than to disregard what a referee has to say. Education is key here. But, some youngsters forget all that when they get on the field. Education.
Unfortunately in my experience this tends to be down to the manager - some I have had this season have been really good, so their team are respectful as well. However, some managers have not been so good, and as a result their team tend to try it on more.
My last U12 game before Christmas was a league match between two teams from the same club. The Colts team (reserves) were brilliant and accepted everything I said, as did their manager. The first team not so much. I had to speak to two players who stood on the ball, and then moaned "how far!" when I walked them back - I'm 6'5", and always give a pretty accurate estimate of 9.15m, but teams of all ages always think I'm moving them too far and moan to that effect.
I also had to warn the manager that if he opened his mouth again I'd have him removed. (At the time his team were losing 1 - 0. Once they drew level and then scored the winner then they were fine!)
The players also had the practise of standing still with a dappy look on their face and both arms outstretched down to a fine art. They soon stopped that once I'd announced that the next one to do so would be cautioned!
Before the game the first team manager asked if I'd be interested in refereeing their team on a regular basis. After the game he was obviously busy because he forgot to ask if I was available!
I find that if you tolerate this type of behaviour then you will find it becomes the norm. If you deal with it in some way - not necessarily a caution, a reprimand can have the same effect in youth football - then you can stop it happening.
As already mentioned, telling the manager that you find this unacceptable will probably work, because you have already said that you know the team and get on with the coach. Get him to do the work for you - it's in the team's interests, because another Referee may not give him the chance.