At age 10 and older, children become susceptible to muscle pulls. When you move up to U-11, you should have your team warm up their muscles before playing. You should have them Warm Up their muscles by light activities such as jogging or slowly dribbling a ball around the field. (Warming up with a ball is the ideal way if it is practical to do so).
The light warm up is important because it "warms up" the muscles which makes them stretch easier & less likely to tear. If you think about it, this makes sense. (Have you ever noticed how all the horses are warmed up before a race?)
Coaches used to believe stretching was good, but an article in the March 2007 issue of Prevention magazine says: "A review of 23 studies found that stretching before an activity damages muscle tissue, which reduces muscle strength and hinders performance. Start each workout by moving your limbs through a full range of motion". Personal trainer Judy Heller is quoted as saying "You want to get the fluid in your joints flowing, so they are well lubricated and move with ease".