I live a few minutes from Webster. Maybe we (meaning me, you, and CivengPE) could get together while you're in town!
Other than that, honestly, Houston in August is not much fun. You will want to be staying inside as much as humanly possible. I'm not sure if we've had a single day in the past month that it didn't at least hit 90F... most days we are closer to 100F... and August is worse!
How old are your kids?
Galveston has some fun stuff to do that covers a wide age range... both of my kids (age 15 months and 8yrs) like the Moody Gardens Aquarium. We haven't done the other Moody Gardens stuff since before the baby was born, but there's lots to do there. Check out
www.moodygardens.com . My daughter and FIL like the rain forest pyramid, but most people think the aquarium is better. The Discovery pyramid sucked as of 2 years ago... we haven't gone back.
If I had a whole day to spend at Galveston, and didn't have to worry about nap-times... I'd skip the beach and spend the day at Moody Gardens. Get an all-day pass ($5 off if you buy it online). I'd probably hit Palm Beach (sort of a cross between a beach and mini-water park) in the morning. After lunch I'd stay inside and hit the pyramids (Aquarium is best) and catch as many of the IMAX shows as possible. They also have a couple of short (12-15 minute) shows with some cool special effects. They are worth catching if you get the all-day pass... but not worth paying for individual tickets. Most of the shows are very dinosaur heavy right now, but most kids like that.
Right next to Moody Gardens is Schlitterbahn.
www.schlitterbahn.com/gal/. They have some huge water slides (with correspondingly huge lines, especially on weekends), lazy rivers, and other water-park things. Some of it is inside, but I don't know how they handle inside temperatures in the summer (it's heated in the winter). Personally, I want to go back, but without the kids. Unless the kids are at least 48" AND daredevils, most of the slides are going to be too much for them.
I'll PM you my contact info, in case you want to get together while you're in town.