M.C.
They are only as safe or unsafe as the person/s using it.
personally, I would let my 6 year on it to jump but I would not let my 3 year old on it to "play". They have to be able to understand the rules of jumping and being safe.
My 13yo ds has one since he was about 6 I think and I don't let my 3yo dd play on it. We will let her get on it sometimes with one of us holding her hand so to speak. ds also can not jump while she is running around the yard because she could go under it.
Anyone that "uses" it is informed of any rules and such the first time with parents permission same as when we were kids except back then we didn't need to ask parents everything.
We got a net but never put it up. It is still packed up in the garage somewhere.