Some bad advice here re goldfish and small / unfiltered tanks. Goldfish grow at least 12 inches and one needs a tank of 180 litres, better a pond. Keeping them in tiny conditions will means they get stunted (organs still grow, body doesn't) and die a slow and painful death.
What I suggest is:
- Set up a tropical tank, all you need to add is a heater.
- 100 litres is a great starter size. Don't go below 60 litres.
- If you buy a tank and stand together (such as a Juwel / Fluval or Fishbox tank) they will be stable and a child won't be able to topple them over.
Fish are great pets, it's a shame some people don't see it this way. DS (3 years) loves all his fish and looks after them very well. He sits next to them for snack times and has a chat with his friends. He watches the babies (got some breeding pairs) and sees them grow. He learns a lot through fish.
If you decide to go for tropical, you can get a nice mix of fish.
A shoaling fish: any of those in a group of ten - neon tetra, ember tetra, harlequin rasboras... (there are loads!)
A bottom dwelling fish: maybe a group of six cories
A centrepiece fish, or some bigger fish: dwarf gouramis, rams, kribensis or livebearers such as platies
And then something like an apple snail, shrimps, otos etc..
There are so many possibilities.
I don't know if you have seen yet, but you need to cycle your tank for 6-8 weeks. Google "fishless cycling ammonia" and you will find some results. Or better, if you find someone with mature media (the sponges in the filter) you can be instantly cycled.
Any questions, please ask. I got 12 fish tanks up and running now, I'm breeding fish and I love it. The whole family is involved and it is a great hobby. Obviously you don't have to be as crazy as I am, but one fish tank alone would be a wonderful addition to any home.