Late august crowds aren't too bad, as most kids in Florida are already back in school and you're still a little early for the Labor Day weekend rush. But, like everyone else said, it will be HOT and HUMID! Bring water bottles (you can refill them for free) and water misters, and don't be afraid to sit down and take breaks in the a/c (shows are GREAT for this!) It's also a good idea (as someone else mentioned) to get to the parks early in the morning, take a break in the afternoon to go back to the hotel and nap/swim, then come back at night.
It would help to know what parks you're going to, how old your kids are, etc..., but these tips will apply no matter what:
*When you get to your first park, head to Guest Relations and let them know that you're first-timers. They'll give you a special button to wear during your stay.
*Some nights, the nightime parades/fireworks shows happen twice. If so, the later ones are less crowded. Be sure to check the schedule before you go- just like there are nights with 2 shows, there are nights with none. And be sure to get seats AT LEAST a half hour beforehand to get a great view.
*Take advantage of Extra Magic Hours. Every day, a different park either opens early or stays open late for resort guests, and the lines are typically GREATLY reduced. The EMH schedule is available here:
http://disneyworld.disney.go.com/wdw/calendar/extraMagicHour?id=CalendarExtraMagicHoursPage&bhcp=1
*Also, there will be a Pirate and Princess Party at the Magic Kingdom while you're there (August 24th). These parties are the BEST way to visit the MK- the tickets are about half the price of a one-day pass ($38.95 for adults if you buy them in advance online), and you can enter the park beginning at 4pm and stay until midnight! At 7pm, everyone who doesn't have a Pirate & Princess ticket has to leave, so the lines go WAY down and you can enjoy all of the most popular attractions, PLUS the special events (parade, fireworks, shows) that only occur at the party AND the whole family can go in costume!
HIGHLY RECOMMENDED!
Have a wonderful trip!