It sounds crazy, but the bigger the tank, the easier it is to keep clean. Gold fish are the worst offenders for leaving lots of mess and should only feed them the size of their eye every day. (that's a good way to measure) They don't need alot of food....the excess food in the tank will cause lots of algae and make more work for mom!
There are other cold water fish out there that are just as easy to care for and are alot cleaner than goldfish. I believe we had zebras for a while in our smaller tank. (I don't think you mentioned you have a heated tank?)
That is alot of fish to have in that small tank.... I think someone already mentioned that it's one inch of fish per gallon of water (or one fish per gallon). If you do end up getting a larger tank down the road (don't let this one discourage you because the small ones are alot of work), guppies are fun for the kids to raise. We raised them for a few years and my kids just loved watching the little ones being born. It was quite educational!! lol They were able to see the whole cycle of life in one tank....it sure made for some interesting questions!