The greatest mistake a person can make when exercising with children is to treat them like little adults. Children are growing and developing rapidly. This means that certain physiological issues must be considered when they are exercising.
Bone Injuries That Affect Growth
When most parents think of children and weight training their first concern is the possibility of it affecting their growth. Contrary to popular belief, weight training will actually improve the bone and muscular systems. It is when children have a lack of qualified supervision that accidents occur. Actually, growth problems are extremely rare when children are given well-designed programs with qualified supervision. In fact, growth problems occur more frequently in dynamic sports such as Tennis, Swimming, and Baseball, than they do in weight training.
Another misconception about growth related injuries is that it will happen without you noticing it. It is actually quite painful because you are actually cracking the top of the bone. It will cease the growth in that bone only so if you get this injury in your upper left thigh (femur), only your left femur will stop growing. Your right femur will continue to grow.
Kids Have Higher Heart Rates and Lower Blood Pressure
A child has a smaller heart and less blood volume than an adult so the child's heart compensates by beating more often (per minute). A child's blood pressure is also less than an adult because this is directly proportional to body size. It reaches adult levels in their late teens.
Children Don't Perform Well Without Oxygen!
Children have a limited ability to perform anaerobic activities due to the limited production of the enzymes required for provide energy in the absence of oxygen.
Keep Kids Cool In The Heat
A child's sweat glands are not yet fully developed so they are less effective at cooling down through evaporation then adults. Make sure they drink lots of water and always exercise in light, cotton clothing.