I got one when I was 10, but I was also involved in gymnastics. I did have a rectangle one, since they are easier for spotting and you can tell where you are a little better. I loved it and was on it all the time and it was always great fun to get my parents and grandparents up on it for a bounce or two. I could usually jump with one other person and we would try and double jump each other, but if they haven't been on one much make sure one person at a time. At the time my neighbors had a 2 year old and she came over and jumped a lot with me and we were just careful on the tramp and had a parent or two on the side spotting, or I would help spot and let her parents jump with her. If you think they would all enjoy I would say go for it, has the 9 year old at least done some kind of gymnastics class to be comfortable on one? I would have them all try one out if they haven't been on one before.
Also make sure you have ground rules- number of people at a time, if they are allowed out without parents, with friends, socks on or off? It is much safer and easier to jump barefoot if you have one of the black ones that are normal for backyard use. I don't know much about the nets that people now use and if they work or not, so that is something else to consider. I used mine for a long time, about 12 years and then since I was no longer at home or visiting regularly my parents took it down so they didn't have to keep moving it to mow.
Just going to ad with all the posts about injuries etc. We used to flip ours upside down when we were going to be out of town to make sure the neighborhood kids didn't come and use it, especially since only maybe one or two had every been on it before. Also of course they can get hurt, but you can get hurt doing a lot of things. I had a cousin who broke her hip when she was little swinging. So make sure they know how to jump and with the little ones you can jump with them holding their hands, so they can go far. And have the rules posted where you can go over them and then oldest can read them.