It’s very important that kids drink water during their training. When we are swimming (which is still an exercice) we also sweat - like any other exercise. However, as we are in water and wet we dont notice or feel it.
Staying hydrated helps prevent the decline in performance, strength, power, aerobic and anaerobic capacity during exercise.
Without an adequate supply of water the body will lack energy and muscles may cramp and a swimmer can feel tired and may have headaches etc.
So, stay hydrated and drink before, during and after you workout.