They are the most dangerous thing parents buy for their kids, over 100,000 kids get hurt each year on trampolines. Please consider something else. The netting is one of the hazards too. They get broken fingers, broken teeth, scars, scraps, etc...when they get caught up in the netting. The American Pediatrics Association is fighting to get them banned from private home use because they are so dangerous.
One story that breaks my heart each time I think about it is where the young man died right there on the trampoline. His mother was standing right there and he was just bouncing. He hit wrong and she heard his neck snap. She will hear that sound the rest of her life. He was not roughhousing, he was not doing wild tricks, he was just bouncing. If he can die so can any other child that gets on one, they are too dangerous, please consider something else.
If you must get one then before you do check with your home owners policy and your health insurance, they may be banned there too. Since so many children and are seriously hurt many insurance agencies are refusing to cover the injuries, home owners insurance is also refusing due to the possibility of someone getting in your back yard and getting killed on it, even if you are not at home.