Cindy,
It depends on your goal. If you are trying to create a prairie that can support some grazing animals, then mowing to improve grass might be the right thing. However, if you are just trying to create a nice looking field of wildflowers, I don't think you need to mow right now. Eventually you will want to have a program of occasional mowing to keep woody plants out. I would probably mow (or burn) in the early spring to cut down trees and shrubs but not interfere with the grasses and flowers. But I don't think it is necessary to mow during summer in order to help the native grasses. I think the long-lived, warm season grasses will slowly gain strength regardless of the flowers and in five or ten years will make up a significant part of the vegetation. For now, i'd let the wildflowers grow an enjoy them. On the bright side, I doubt that the occasional mowing will kill any wildflowers, it will merely stop or delay the bloom. many will try to rebloom this summer, while others might just grow leaves and roots, and not try to bloom until next summer, but you probably didn't kill many (maybe not any). If you feel that you want to mow, one good option might be to experiment a little. Perhaps some portion of the prairie you could mow once or twice in the summer, and another portion not mow, and a third mow only once in may, etc. This should prevent killing any wildflowers by too much mowing, and give you a feel for how the plants respond to mowing. I think most people who plant a prairie just for fun or for restoration rarely if ever mow during the growing season. In the midwest burning the prairie is a popular option because it replicates a natural process that is pretty much gone now - there aren't too many wild prairie fires in most of the US these days. I think with smaller prairies and other parts of the country where burning grasslands isn't as common, most prairie owners just mow to remove trees and brush. mowing probably doesn't work as well, and you might have to go around and pull or in some other manner kill trees and shrubs occasionally. As for what to do now, I'd just let things grow and pull any invasive weeds (like canada thisle).

There really isn't a right or a wrong. In the long run the grasses eventually outcompete the flowers anyway...in several years you will be asking yourself, where did all this grass come from.

The mowing of an immature prairie is primarily to keep the annual weeds from going to seed. If you have the time and are willing to do the labor, you can manage weeds without mowing, its just more work. Hand picking off the flower heads or spot treating each weed....

Your flowers will come back - they are perennials...