Hi, Melissa! There's no way to sugar coat this - it is going to be hot in August. Temperatures in Walt Disney World in August will be in the 90s with high humidity, and you will probably be treated to a shower in the afternoon each day. Unfortunately, the shower does little to cool things down. On the upside, there are plenty of opportunities to be in the air conditioning - in the shops, restaurants, and on quite a few of the rides and attractions. The lines at the end of August tend to get lighter, although you may not find that as much the case with the offer of free dining.
We usually go at the end of August, and we just plan to be hot. Keep lots of sunscreen on everyone, and have them wear a hat and sunglasses. Dress them in light, breathable fabrics and in light colors. Keep the kids well hydrated - lots of water or juice, whatever they will drink.
Another lifesaver for me has always been the clip on fans that you can attach to your stroller. They now sell them in the theme parks, but I've gotten them at
www.onestepahead.com . I always have two of them, and they run about $16 each. You need a screwdriver to put the batteries in, so bring a small one with you for replacing the batteries. With two of the fans, you can point one at the baby and point the other at the seven year old walking next to you, or up at the person pushing the stroller.
There are definitely less hot and humid times to go, but you can manage! Plan breaks to cool off, and you should be fine.