This isn't the NFL

Kids experiencing full contact football for the first time need to understand a few points about the game. I usually spell it out for them at the beginning of the season and reemphasize them throughout the season.
When you watch kids play football on the playground often its everybody out for a pass and rushers required to wait the agreed upon amount of time ("Mississippi's" in my area). Ninety-nine percent of the plays are passes. Blocking is nonexistent.
Kids need to realize some of them will play the line and you have to convince them they are very important positions. It really helps if you truly believe that and the kids will gauge how much you believe it by the amount of time and the attention you give to teaching linemen in practice.
Kids need to know that they will get bumped and bruised. Begin teaching the difference between a bump, bruise, or scrape and a real injury. Be patient with kids on this point and treat all their concerns as injuries (to be on the safe side). They will begin to see the difference with experience. Its been interesting that every season when full contact begins, one or two kids decide they don't want to play anymore. I encourage kids to stick it out and nothing I do is designed to "weed out" kids, but regrettably some decide the game isn't for them.
Most plays will be runs. A lot of kids want to be wide receivers. I tell them "that's great, but we don't have a position called wide receiver." Maybe some of you can move the ball consistently at the middle school level with the pass. I've found if we pass more than 5 times a game we're in trouble, because we can't move the ball on the ground. In one season (5 games) we threw 18 passes 12 were completed (9 for 2pt conversions and 1 touchdown). We were 4-0-1 that season.
As much as we as coaches understand it, the kids need to know that we are not going to play run and gun football. I'm not taking the fun out of the game for them, but simply trying to give them a realistic picture of what the game will be. Then I work on making the game, in that context, fun for them.