The tank NEEDS to be cycled. Every tank does or you will just waiste your time and money putting fish in and just watching them die. Its a deathtrap waiting to happen. If you are planning on doing a fishless cycle than you need a source of ammonia. Hardware stores sell them, the product ' boots ' has a pure ammonia source. The ammonia in there will help get the cycle started...
But if you want the easier route, there is cycling the tank with some fish. I would not put in goldfish. You are doing a tropical set up and Goldfish DO NOT belong in tropical tanks. Some good hardy fish for cycling a tank are some zebra danios. They are very very hardy. Some platy's are also good too. Get a few of those and watch the water stats. The tank should be around 75-80 degrees.
The point of a cycle is to get bacteria in there, making bacteria colonies. The bacteria than eats the ammonia which than turns into nitrite. Which than turns into nitrate, which is a less harmful thing. Nitrates in a tank are good from 20ppm and under.
Oh i wouldnt recomend doing some cichlids, they are harder and more spendy.
You say you have kids and you would like it to be childish sort of?
I would go with some colored gravel than. Depending on what your kids like, blue usually suits well. Than some good ornaments that your kids will like. Live plants also look very good, and they help keeping the nitrate down a little bit.
I hope this all helps