You'll want to check these articles - they'll help you understand what's going on with your tank:
http://www.fishlore.com/FirstTankSetup.htm
http://www.fishlore.com/NitrogenCycle.htm
It sounds like your tank was cycling - an ammonia spike is part of the cycle. The cycle can be hard on many fish - i prefer the fishless cycling method myself. You can also buy a product called biospira which will instantly cycle your tank and make it healthy for fish IF you can find it locally or want to pay overnight shipping on it from an online source. (It needs to be refrigerated because it's live bacteria.)
If your pleco is a common pleco, they might not have told you but those grow to 18" and sometimes larger. A 55-gallon tank (or bigger) is recommended for one of them. They're also heavy waste producers and put out more waste than you'd believe. For a 10 gallon tank, otos would be better for algae control.
The fish could probably eat a bit more food safely. You could feed them a pinch AM & PM (more fun for your daughter and better for their digestion system).
I'd recommend returning the pleco as he'll get to big and keep your tank dirty.
Also it would be good to get the API master freshwater testing kit (or the testing kits for nitrites and nitrates) - the master kit is on sale at petco now for 19.99 and i think petsmart might still have it on sale for that same price or you can find it cheaper online at
http://www.drsfostersmith.com/Produc...&N=2004+113074.
You might not want to get more fish until the tank has cycled and you'll want to stick with fish that are happy in the same temp. range as goldfish. Fantails are cute and fun for kids.. they also had yesterday at our petco, these funny little goldfish that look like they swallowed a pingpong ball that might be fun for your little girl (LOL - they'd be fun for me too, i want some myself.

) she could have fun picking out new goldfish from the many small varieties. Find out the adult size of any fish you're considering. The rule of thumb is 1" (adult size) of fish per gallon so your tank would hold 10" of fish.
Another thing i'd recommend would be to treat the tap water with NovAqua+ or Prime. oh.. if they try to sell you a product called "Cycle", don't buy it. it creates mini-cycles and we found from personal experience that it kept our tanks from really cycling. Best of luck and we'd be glad to answer any questions you have.
EDIT: I see you're a new member.

welcome to fishlore - this is a GREAT community for help/info/support and there are a lot of really great people here with lots of experience. we've been learning a LOT here and made some new friends too.
