A lot of the parents I work with still obviously want to take their kids. And I'm certainly not saying that you shouldn't go ... just keep a few things in mind. I've suggested that maybe you don't go on the hottest, sunniest days ... these tend to be the busiest. I'm certainly not suggesting you go during monsoon season, but cooler more overcast days might be a good idea.
If your child wears ear phones for sound, don't forget to bring those along! Just because you're outside, it doesn't mean it can't get loud. Schedule in some quiet (as quiet as you can get) breaks throughout the day - you don't have to go go GO!
Watch your child's reaction to those big fluffy characters - he or she may love them ... but he or she may hate them. They're not for all of us ... they're not for me!
You know your child and if your child likes to ride the rides then think about the lines. Most amusement parks that I've had experience with will allow your child to skip lines with proper approval. Usually you need to do this at least a few days in advance (but I don't know anyone who has just shown up with a note the day of the trip). Ask your doctor, OT, SLP, ABA coordinator, etc for a letter stating your child's diagnosis and why an amusement park may be overwhelming. In the past, we've done this with a number of 6 Flags and Disney World and have been successful in acquiring a pass for the child and usually the entire family. Cutting down on wait time in lines may help make this trip a more positive experience for your family.
Lastly, talk to your child. Some kids benefit from being able to prepare themselves somewhat for an upcoming experience that may prove trying. What will best prepare your child? Will a social story help? Will trying to answer any questions or reassuring any concerns they might have help prepare your child?