A few things to consider, if you have not already.
How sure are you that the current school will become a "bad fit" next year or within the next few years for DS5?
Is it possible to leave him there and move as needed later? Does a sibling get preference later as far as admission, despite the waitlist?
There is a value to the financial savings for DS5 until such time as he needs a private school.
I would not keep the older child in a bad fit environment just to keep younger child in it. If logistically it is possible to handle two schools, I'd look hard at that option.
