Although I'm probably a bit late posting our experience to help you, it may help the next family that stumbles upon this post.
Our youngest is also hypotonic among a few other things. She is in PT, OT and did 4 months of speech.
We took her at 21 months and she really freaked out on half the rides. We didn't realize before we went that she also had sensory problems.
However, we're going again soon. We split up and take her 3-year old sister on rides while she runs around outside.
I worry more about her hypotonia and drop foot. With hypotonia, she gets slung around a lot more. She struggles to walk without falling; long lines can be treacherous! Splash pads are an accident waiting to happen. Although we have not gotten genetic testing, she appears to have an overgrowth syndrome. We had to avoid the temptation to take her on rides that she was technically tall enough for because she wasn't strong enough for them.
Rides I won't take her on again include noisy, dark, rides that sling you around. No Pirates of the Caribbean, little mermaid, finding nemo, buzz light year, toy story, Winnie the Pooh, haunted mansion, fireworks (even from the wilderness lodge)
Rides she enjoyed include dumbo, Aladdin, train, petting zoo, carousel, baby center, most everything at Animal Kingdom except the dark shows. Characters were hit and miss.
Legoland is spectacular for this age group and this set of special needs. It's only an hour drive from DW. you park strollers right by a Lego play area. The kids play with Legos while parents wait in line. Legos are good for OT/fine motor. The rides are slower and surrounded by Lego duplo animals, which we use to learn new sounds (you will learn animals sounds are pushed heavily in speech therapy). Plus they have toddler size playgrounds both inside and outside.
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And a huge thanks to you for posting this! I think my SPD girl is probably a little older than yours. She's 3 now and everything you commented on here is very similar to what we deal with. We also haven't had genetic testing yet, but Boston Children's suspects that it may be the cause. Lego land sounds great but yikes, legos would be very frustrating for my little one! She struggles too much with her fine motor skills to play with the duplo legos without rolling in tears after a few minutes.