Treetops is a nice park with nice pool but I don't remember any playground or activities for little ones. Their rules are strictly enforced.
I would try The Vineyards at Grapevine Lake. No pool but they do have a swimming beach. They are open with limited sites due to flooding.
Things to do:
Fort Worth Stockyards (spend the day). See horses, cattle.
Dallas Zoo & Fort Worth Zoo
Six Flags (flooded now)
Museum of Science and History
National Cowgirl Museum
Children's Aquarium
North of Dallas in Frisco is the Museum of American Railroad.
Frontiers of Flight Museum & Cavanaugh Flight Museum
Take the train into Dallas to Dealey Plaza.
Aboretum and Botanical Gardens
you will want to limit outdoor activities during the afternoons as it will be hot!