I took my grown kids to it so we could be "kids again." Dont it twice -- several years apart. Loved it. The advice about not going early to be so tempted by all the stuff is good. You could go to a toy or party store and buy a sword or hat in a bag, don't let them linger in the store part, and bring out your surprises at your table area.
World Acquarium: We went and it was a nice alternative to being out in the hot weather at the zoo. But we didn't go to any of the demonstrations which sounded good and would have added a lot to it.
Southlake has a "bounce house" for open play at times and birthday parties (expensive). We went in to look around. Pretty cool. My 3 year old little granddaughter followed a little boy up to a slide which I would have thought she'd need to be 5 to have the nerve to do that. She went over and over and over again.
Disney had a pirate and princess party at movie houses earlier this summer. Pick a pirate type movie to go to or play on the biggest screen you parents have, have a summer party with his friends, and invite them to dress up as pirates. Before the movie, have them walk a "runway" and pose in their bestest and most fierce pirate pose for pictures, alone and together as a group. If at home, put some pirate out.
The Flower Mound community activity center has a cool pool with slides for the kids.
Go take in the Gaylord Hotel inside. Looks like your outside but it is air conditioned. Very cool looking for adults and kids alike.
There is a Bed and Breakfast out in Mineola, TX that is way nice. It is a 1.5 hr drive. (Do they allow kids? Unsure.) One suite has a twin on the second floor balcony. It's called Fall Farm. Some of the most beautiful rooms you'll ever see. there land goes forever, lots of room to see, A shaded patio out in the middle with two facing porch swings. A huge pool. She keeps snacks and fruit and drinks on hand free of charge for you during the day. Gourmet breakfast. Great breakfast. Show your boy how to socialize with strangers at the breakfast table. A masseuse (by appointment). Scads of books and Magazines. Interesting people to see where they're from, what they do, how they found this place. It's $185 for a huge two-three room suite. You will not want to leave there but you'll want to go into town to see the old timey stores, restaurants, antique stores (closing early-ish on Sunday). Tyler's near there too. They had a private zoo which looked amazing when I peeked in. Tyler has some flower festivals during the year for their roses and azalias. Don't make the mistake of staying only 1 night. It's hard to leave the peace and beauty and adventure out there!