Stretching helps to loosen tight cold muscles as well as promoting longer and healthier muscles and tendons. By increasing their flexibility and strength you promote a healthier athlete that is less likely to get injured. The warm up should take place with the stetch so that your kids are prepared to go to battle as prepared as they can be. Stretching at the end of practice is also becoming more and more common and I personally always stretch at the end. Make sure you are warming up, stretching and cooling down every day.