I have met parents who believe trampolines are fine and allow their children on with minimal to no supervision. I have also met parents who are terrified and never allow their children on for fear of injury. I am of the mind that there is a balance. There is no doubt that trampolines are dangerous if not used properly. Even with the current availability of safety netting, injuries occur. It could be as little as scrapes and bruises or as severe as sprains, broken bones, or paralysis.
Trampolines and Young Children
First of all, it is recommended not to use a trampoline for children under aged 6. As their spines and bones are still developing, their necks are "soft" and you will notice that their heads are jarred much more frequently and severely than an older child with better neck control. However, general child safety at this age are:
1. Always supervise your children
2. Use safety netting to prevent them from jumping off
3. Only allow one child per time to jump
The main thing to watch for if two children are jumping at the same time is their weight. If one is considerably lighter than another, they are far more susceptible to injury.
Trampolines and Older Children
To be honest, I do not fear younger children on the trampoline. Most likely, your injuries are going to be limited to scrapes and bruises. In older children however, you have accessed a whole new level of intensity. Older children weigh more and they do more. They want to do flips, jump off the roof onto the trampoline, double bounce each other, etc. To me, this is the age to watch for and have strict rules that have to be followed.
Let's be clear that any safety rules strongly advise only one child jumping at a time. However, let's not be naive. There is no way that is going to be happening. The whole fun of a trampoline is jumping with other people. So... it comes down to etiquette and safety. Your older children need to be aware that if anyone feels out of control, unsafe, or scared, to stop bouncing them and let them off the trampoline. If someone is smaller than them, they should be more careful and not bounce so high. Never should there be more than two people jumping at a time.
In conclusion, I must confess that I have a trampoline. I have a 3 year old and a 2 year old that jump on it... together. It has safety netting (which I tied to the inside rather than outside so that they cannot access the springs) and they jump on it all the time. Surprisingly the few injuries that have happened have been things like bonking heads against each other, one pushing the other off the trampoline, and "rug burn" from hitting the mat wrong. However, my children jump on it for hours a day. They get out so much energy and sleep like "babies" at night. They are getting exercise rather than sitting in front of the TV and it is something I enjoy doing with them too.
So if you are considering getting a trampoline for your family, weight the pros and the cons. Consider the ages and weight of your children, consider how much time you are willing to supervise them on and definitely get the safety netting. The fact is, your children will get hurt on the trampoline. You must decide if they (and you) will be cautious enough and if that risk is worth the added benefits of this backyard toy.