We went to Orlando with dd7 ADHD, asp, last summer and it was great. Guest assistance pass was a huge help, bring a letter rom the consultant to say the diagnosis and outline particular issues (crowds, noise and queuing were on ours). We didn't use it all the time but a fair amount and it was great as we were there over 4th July break so absolute peak!
Get an idea in your head if you can of e outline beforehand and grab maps as soon as you get there. Important things are water fountains (bring bottles to fill to make you drink as you wander), toilets, food spots, quiet places for a break and guest assistance locations. Orlando had a place for disabled, incl asd types, to go for rest and quiet if needed. Big help when things seemed to be getting too much to know it was an option, although we managed by just finding quieter spots in the shade or going for food. KNOW to take breaks as it is very easy even for NT people to get overwhelmed. And bring bottles to fill at water fountains for while you are wandering, to keep hydrated (save fizzy drinks for meal breaks, but you can also get tap water free in all restaurants).
Prepare the DCs for possibly not doing everything, long queues or not suitable or not open today. Check beforehand if there are rides they can't do, especially on height, so those are knocked on the head before anyone gets excited about them.
We told dd about it beforehand, as she needs to know the plan. And got her a kids Disneyland book. But she's NOT into princesses or girly things so didn't bring a dress or anything. Although we did end up buying a pin everyday instead.
We also had lots of things planned out, in case we needed time out, weather was against us or whatever. So I had colouring pencils with free printable Disney pictures in my handbag (always have colours and pictures of some sort, and puzzles), and we brought her DS game as well to play if q,s were very long or to just zone out in quiet restaurant corners at times. And we talked beforehand about expecting q,s and heat and crowds.
And we learned after Big Thunder Mountain to eat AFTER big rides, into before them (felt queasy rather than actually sick). And also to pace the BIG rides with quieter ones and gentler ones. But we still had excitement, like "daddy too scared to go and mummy very scard on tower of terror, but I wasn't at all!" (Not that you could call her not scared when you see the photo of her gripping my arm and screaming!
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If I can find them and figure out how to link them now (I sorted it all at work last year but MN now blocked there), I will post links to a few websites that were useful. Mostly about Orlando, but lots ASD and Orlando which may be useful in general re Disney. And staff were all really helpful all the time.
Enjoy!