Larger children jumping out of sync with smaller children may result in injury or ejection of the smaller child from the mat. Trampolines can cause severe injury in children. The most dangerous part of the trampoline is the mat itself. This is where children tend to fall and get hurt. Ejection injuries, while severe, are much less common.
Parental supervision is paramount in limiting risk to children on trampolines.
The best thing the parent can do is keep the jumper in control and to limit the jumpers to one at a time on the mat.