I am going to say this and then be done.
Over 168,000 kids get hurt on trampolines each year, sometimes they die. most of the time they get broken legs, ripped out teeth, broken fingers, and that type of thing. They can break their backs and necks and other things that don't heal too.
The main thing that causes these accidents is kids jumping at the same time. So if you want your kids to jump at the same time please just be aware, that is the most dangerous thing you can let them do other than do flips and tricks.
The American Pediatric Association would love for them to be against the law to sell to homes. They are the most dangerous thing a parent can buy their children.
That said....
make sure your home owners insurance does not prohibit them from being on the property, if the kids get hurt do not be surprised if none of your insurance will cover any part of their bill too. If a neighbors child gets on it and they even get a scratch those parents can sue you for everything and get it. If their child gets hurt in a more serious way you will be paying out of pocket for every bill they have too because your home owners Will not cover any injuries on a trampoline.
So other that setting your children up for serious injury and yourself for law suits from neighbors and other family members trampolines are a great deal of fun.