Hi M.,
You don't say the ages of your children, but here are somethings around Fort Worth:
Zoo - absolutely incredible. Hot, but you can duck into a few air-conditioned places on the way. http://www.fortworthzoo.com/
Miniature Train: Another mama suggested this. You can get on right outside the zoo. http://www.fpmt.us/
Another mama also mentioned the Bureau of Printing and Engraving. If your kids are old enough to understand money (probably ages 6 and up?), then this is a very cool sight. http://www.moneyfactory.gov/locations/section.cfm/25
For baseball, we prefer the minor league Cats games. Much more family-oriented, not a huge stadium, not so loud, etc. All the great things about minor league, and the field overlooks downtown FW. Oh, and of course it is less expensive than the Rangers. http://www.fwcats.com/
You also don't say where your family will be coming from. If they are coming from very far away, be sure to take them to a BBQ restaurant. There is also a burger joint called Kincaid's that was just named in Gourmet Magazine (national press!) for one of the best burgers in the country.
Entrance to the Kimball Museum is free and the building itself is a work of art. Very famous building by a very famous architect. Other museums have entrance fees. However, you could spend a couple of morning hours at the Kimball and walk across the street to the Modern and enjoy a delicious lunch overlooking a reflecting pool
Have fun!
M. Elizabeth