V.P.
Gosh... That's a tough one... keeping a 4 year old boy and grandma happy in Texas in July...
There's the new Lego store in Frisco. They often have special events (http://stores.lego.com/en-us/Frisco/events.aspx). Obviously, grandma's probably not into Legos, but you could combine it with a walk around Stonebriar and lunch.
Lunch at Rainforest Cafe in Grapevine.
Stroll around Gaylord Texan. They've got SummerFest going on through early September, every Thu-Sun. Includes mini train displays, cowboys on stilts, jugglers, etc. Parking is $10-$12. Then you could stop and have ice cream inside the atrium.
Maybe a trip to Bass Pro in Grapevine? They have lots of cool displays for kids (aquarium, stuffed animals, tents set up, etc. You could even do this in conjunction with the Rainforest lunch.
The last option is outside of your noted budget, but thought I'd throw it in there... Dallas World Aquarium is a fantastic place, both for 4 year old boys and for grandmas. However, it would run about $50 for the 3 of you, and they're not an organization that generally offers coupons.
Good Luck!
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