A.D.
Actually the Ft. Worth science museum has some of their children's exhibits set up in the Cowgirl Hall of Fame, your ticket gets you into both parts they change it every few months so I'm not sure what's there right now but the last time we were there there was a play grocery store, a play house, some science stuff and some space stuff as well as a few other things. Enough to keep my 3 year old occupied for well over 2 hours.
The Dallas World Aquarium is supposed to be pretty great, it's on Griffin St. near the West End.
If the weather isn't too bad the Stockyards are always something free to see. You don't say whether or not you've taken them there before. The Money Factory in Ft. Worth is worth a tour as well. They run free tours daily during the week, just go to their website for specific info moneyfactory.gov.
Hope this helps!