There are several things you can do. If you don't mind a little bit of a drive there's a place called the Cowgirl Ranch in Westlake (just off hwy 114) and it's FUN! My son had his 4th birthday there and we had a ball.... invited 12 of his friends, they fed and rode horses, played games, I provided manwiches and baked beans, chips and cake...she provided lemonade and something else...made it totally into a cowboy day - My son's birthday is May 28 so I know what you mean about it being hot!
Another idea is Pump It Up... those are always a big hit... as is The Main Event in Grapevine...
For his 6th birthday we had his party at the Fort Worth Zoo...it was FANTASTIC! It was like $200 and he got to invite a ton of friends, they all got free admission to the zoo for the day...they bring in live animals from 4 different groups -- snake, bird, mammal and amphibian... it was WAY cool... all the kids got a souvenier and I provided pith hats and a waterbottle for a souvenier/party favor. This year he turns 7 and I rented out a climbing gym since that's his favorite thing to do now. :)
Good luck... we as moms ALWAYS make it WAY harder than it has to be!