G.D.
I have on my list - Grapevine Mills Mall just installed an aquarium. I haven't seen it yet but sounds good.
Based on the ages and no price limit, you might go see a play at the Dallas Children's theater or the Richardson Eisemann. You can also go to Slappy's at the Galleria and let the kids play in the playcenter after. We took my in-laws to the Nasher Center.
The senior might also appreciate the flight museums. The one at Love Field has a play area. The Cavanaugh has both indoor and outdoor displays - interesting for kids and seniors. There is a car museum in Richardson on Campbell Rd.
There is also a soda fountain in Dallas in one of the drugstores. Think ice-cream and frozen custard too. Enjoy!