L.H.
Picnics (have them help prepare the meal) then go for a walk. You can make leaf rubs or see how many birds they see. There are lots of splash parks in the Dallas area if you don't want to do the water park. I second Hawaiian falls and wouldn't take them to hurricane harbor at their age.
The Dallas and Fort Worth zoos are great. The science museum in Fort Worth is amazing. I've heard the Perot museum is awesome as well but haven't made it yet.
Enjoy time with your sweeties. They are blessed to have you!
L.