As for low cost tank set up, I know Petsmart was running a special on a 10 gallon with hood and light for $30, not sure if it's still going. It did not come with a filter, but that allows you to use what ever filter you want rather than the way to small ones included in most the kits.
When I was married my step daughter wanted a tank. I had 3 running at the time and told her I would be happy to help her set one up, but she had to know what she wanted and how to take care of it properly first. She was 7 at the time. We first went to the pet shop and she wrote down the names of the fish she liked best. We went home a read about the different types. She narrowed it down to Glofish pretty quickly. She knew exactly what size tank she needed, how to do a water change, and how much to feed the fish. I trusted her with my fish more than her father! Unfortunately her dad and I split before we could get her tank established.
This just goes to show that with proper guidance children CAN learn whatever they set their minds too. If you want your kids to have a tank, help them learn. It will bring you all closer together and you know that you are teaching them right from the start so your tank doesn't end in tears.