Get as big a tank as you can afford whether its for goldfish, a betta or tropical. Goldfish are quite dirty fish so actually need more space than tropical. I would advise a tropical tank, with adequate filtration and a small heater. Then get some easy to keep community fish like Platys. The advantage with livebearer fish like platys and guppies is that they give birth to live fry and if you have a few plants in the tank a few of the babies will survive and grow up. A nice intro to the facts of life for DC.
My son has a tank in his room, with a few platys and guppies and was thrilled when he saw the babies.
You must keep up your weekly or fortnightly water changes though, when you clean the filter, no matter which fish you buy. It is quite true about setting up the tank before hand to mature the filter but there are several ways to do this.
Get a good book on the subject and read the fish forums. Can I recommend initially Fishkeeping for Dummies, or the Tankmaster series on Coldwater fish, or tropical fish?
They arent as easy as you think to keep, I started with just one tank, realised it wasnt big enough and at one time I had 5 tanks running as I had read up on the subject and realised I needed more space. Do get some live plants too. I used to think you could just put a goldfish in a bowl and clean it weekly, but these days that has been found to be cruel. The bowl distorts the fishes view on the world, its cramped, he can only swim round and round, and he has to live in his own excrement if you dont have a filter.
Avoid the biorbs though, unless you want some dead fish to explain to the children. When I downsized my fishkeeping, I did it so I could get the family a much wanted puppy. Despite what you think, the puppy is less trouble!!
If you want to know anything else, please get in touch.