Danios prefer the warmer water too but you can have a school of White Cloud Minnows in there. They will be fun to watch and are very pretty. I believe there are coldwater loaches you might be able to add too though I'm not too versed in them.
Your water is going to have a good amount of oxygen already in it since colder water is more sufficiently oxygenated. The filter will be sufficient enough at moving the water surface for the exchange.
Do note that you will be buying a school of fish to plunk you through a cycle. Not sure if you've read the cycling sticky or not but in the case of your excited son, it seems the only way to go. Is it preferred by all? Nope, but it is done nonetheless and we can work with it. For starters, I would start with only 3 Minnows and have them work through to the end of your cycle. You will be doing a lot of testing and a lot of water changes and that all can be explained here:
http://www.aquariacentral.com/forums/showthread.php?t=84598
Finally, Congratulations of doing the best thing possible, and that is coming to do your research before actually getting the fish. You will find that a lot of LFS people are in for the money, rather the fish and will find that in time, you will be able to separate the good info from the sales info and you will find your own reputable Fish Store buddy. Good luck with your tank and welcome to the board. We hope you enjoy your stay and welcome you and your son to the hobby.