There are lots of combos for what you can do with a 10g, depending on what you want. You say a community tank? Would you like multiple fish such as a school of rasboras or a variety of guppys, or one larger fish like a betta? There are other interesting things you can add, like a snail or shrimp that your 5 yr old might like.
Live bearers such as guppies might work,
Small tetras such as neons, would work,
cherry barbs would work,
rasboras would work (probably not the swordtails as they are larger)
You could get a dwarf gourami and a small group of one of the smaller corys.
As for the salt, there are some fish that are brackish or full salt water that you probably don't want to mess with as they are more work for a beginner. For the majority of freshwater fish, you want to keep salt in the tank only as needed to get rid of ich and such.
To get some ideas of good tank combos for a ten gallon look around the newbie threads and take a look at what other people have suggested. Just remember that it is better to understock then overstock, and to look at the full grown size of the fish when you put them in. Also to remember that fish use different parts of the tank, so don't put in too many that use the same area. In addition, many of the smaller fish are schoolers, and need to be in larger groups, which limits the number of types of fish you can have in the tank. But a school of fish can be fun to watch!
