Children's perception of severity of exercise is less the younger they are. Adults sensibly stop when exercise is too hard, children tend to press on. In certain circumstances this can lead to them over doing it! The safest way to avoid the problem is to plan shorter periods of activity, where feasible, than would be the case in an equivalent senior squad. Drinking water or squash in the breaks during a session is a sensible procedure for all athletes, in particular for juniors.
The most important factor to watch out for with youngsters, is when they are not responding as usual in a training session. Don't always assume it is lack of effort, there are many reasons - especially with the junior girls - why they are below par and not up to their usual active self. Sensitive handling at these times ensures that the youngsters return each week in the knowledge that their problems have been noticed and discrete action taken.