daisybel1 just tell your mum what's happening because there really are times when people just have to accept 'things are going to be different this year'. I wouldn't even beat around the bush, I'd just say, 'its happening because of x, y, and z, and on this occasion their needs outweigh yours'. It would take a real horror of a granny to not understand, and if she doesn't then all the more reason for saying, 'well I'm sorry but it's what's happening'.
As for granny going? Well I was in the Florida Disney with my brother and his wife for 2 weeks last year, it was magical by the way, so magical we are going again this year for 5 days after our Cruise, and I did see older people being taken around in wheelchairs but very few of them looked happy. I think they had perhaps been taken because no one wanted to leave them at home because of a situation similar to yours, or they had gone along thinking they would like it. Its a really tiring day when not in a wheel chair such is the level of excitement and plain daftness , a wheel chair wouldn't lessen how tiring it can be.
As it is you have a disabled son in law, a not very well daughter, a newish baby, and a squad of kids - I don't know how you'd add granny in to the mix and this comes from someone who regularly does Euro Disney with just one disabled person. And if its Florida you're going to, my goodness its huge and if I remember correctly the entire Euro Disney park fits into one of the Florida Disney Car Parks. It took us 2 solid weeks to do and see everything and it was the best fun I had ever had. It was Disney morning, noon, and night. I really only went along to spend time with my brother and his wife but all my reservations vanished the moment we drove up to the entrance and I loved it.
I hope you get this sorted. I do understand how it is to have to take a much loved older relatives needs into account, my very elderly sister in law lives with us but the reality is that unless the person has specific needs they really should be able to cope whilst others have a break, and not just when someone else in the family has a disability.
One last thing however - be warned that that Christmas time is probably the busiest time of the year to visit.
Here's to you all getting away on your holiday