I have 3 young children and have kept tanks for many many years. So I have put much thought into this topic as they have been interested in my tanks since before they could walk.
To answer your questions 1 FW
is easier than SW
. 2 tank stands sold at LFS
are completely sturdy enough the tank Isn't going to tip over. the only thing you are going to need is locks for the cabinet. The child proof braces that go through the handles is sufficient. But what ever you do don't keep the chemicals in the cabinet keep it in a place outside of the play room.
Before I tell you my decision of what I got for my Kids I will tell you why.
My criteria for selecting fish
1. I wanted fish that where hardy and lived more than 5 years. Because I don't want to have the "why did my pet die" conversation with a 3 year old. Unfortunately their great grandparents died so we had the death conversation already.
2. I wanted something colorful, brown fish aren't that exciting to kids.
3. I wanted active fish, again a predator that lays at the bottom isn't much fun.
4. I wanted something easy to care for because kids don't leave allot of extra time and I already have enough beings to care for.
5. I don't want aggressive fish. Even though I love central american cichlids I don't want my kids to be screaming "help daddy joey's fish is beating up my fish" Kids do tend to pic out which fish is whose and name them.
So my decision was Gold fish. I feel Goldfish meet all of the above requirements. We currently have 2 and they have been alive for 3 years and my kids love them. I know goldfish are common but they are common for a reason, beautiful colors and hardy.
So my suggestion to you would be get a 55 or 75 what ever your preference and get 4 or 5 gold fish.
PS don't get fooled by these shows like Tanked or Fish Tank Kings where they set up these beautiful reefs in kids centers and playrooms. They are great ideas but not very feasible. Kids banging on glass will kill many types of fish. Besides most of those tanks are maintained by professionals or they die.