Six Flags, unless you think they might be a little young for that.
Lunch at the Rainforest cafe at Grapevine mills mall. My daughter always loved that.
Planet Pizza in Plano has indoor small amusement rides that are fun. I have not been in YEARS so I am not sure it is even still open.
Dallas Arboretum....pack a picnic or buy lunch on the premises. Beautiful and they usually have an area fun for children
Science Place at Fair Park
Mesquite Rodeo if that is going on right now.
Dallas and/or Fort Worth Zoo (I hear Fort Worth Zoo is better, I have never been)
Hope those help a little.