If you can really afford it (take an honest look at the sacrifices and whether they are sustainable in the long run) I would move both.
As platypus says, I’d also take an honest look at how much worse the situation could really get for your DS and whether the new school might be as good a fit for him as for DD, but I have to say that a school deciding to meet the needs of intellectually disabled children with violent tendencies by moving them two grades down would be a huge red flag for me!
Agree that some choices can’t be up to 5 year olds. If you feel that the more expensive school suits him well and the sacrifices don’t impact him (or other family members) unduly, I’d move them both.
