Although I realize this may be coming a little late now that summer's officially over, my guess is you could still use ideas for things to do in Charlotte for kids year-round. One of the free things Charlotte has for kids that's offered on an ongoing basis is ImaginOn in uptown. They've got hands-on activities, books and a variety of media for young children - all of which you can take advantage of at no charge.
If you want ideas on kids events in Charlotte NC and surrounding areas that happen every weekend, try this website:
Charlotte NC events - concerts, theater, festivals, kids events & sporting events
They've got a section just for kid-friendly events as well as a different section for free events that often has little festivals sponsored by local groups that are free or low cost for kids.