Hi Melissa, Thanks for your great question! I can understand why you would want to bring your daughter to a Water Park in August.
I've done some research for you, but some of this depends on a couple factors. First, is she confined to a wheelchair at all times, or can she get up and move a little on her own? Can she swim and would you feel comfortable with her going on a water slide?
Wheelchairs are welcome at both Disney's Typhoon Lagoon and Blizzard Beach Water Parks. Personally, I wouldn't recommend Typhoon Lagoon as it has a lot of stairs. They do have the lazy river, which you could get a double raft and ride around it with her. Depending on her disability, I'd be a bit concerned about safety in the event that the raft flips.
Blizzard Beach actually has a "Ski Lift" for wheelchair access. The Park is themed after a ski resort (on a hill). So, the "Ski Lift" is part of the theme and it is wheel chair accessible (it would be bringing her up to the slides at the top). Again, you would need to decide if her disability would allow for her to ride the water slides safely. They do have "Tike's Peak" play area for kids.
I would recommend that you call 407-W-DISNEY. Get an operator on the line and ask to be transferred to the Blizzard Beach Guest Relations department. You can then ask Blizzard Beach about any specific concerns that you may have.
Hope this helps and that you have a Magical vacation,
Kaylene