L.S.
The Coppell Public Libary does a number of free things throughout the summer. You can check out their calender online. My 2 1/2 yr old loves going to the library if only to just play and read books.
The aquatic center is also pretty cheap and a good way to cool off. I think it's $4 for residents and children are probably cheaper.
Kidsmuze is an indoor play area that your children might like. I don't know how much they charge for kids though. I think they have gotten rid of charging for adults now.
Irving Arts Center is also doing a number of family/children's events. I think the tickets were around $3 for some of them. You can also look at the galleries for free I think.