I think people here have mentioned at least being able to get a seat that raises the height of the toilet up, but I'm not sure that I've heard of bars also being provided. I would not expect them to have bars for the tub. I believe you can ask for a portable tub bench, but I don't know if it would have enough in terms of stability for the member of your party who needs them. Something like those portable bars that attach to the walls without being screwed in would be far too much of a liability for WDW and, personally, I'm nowhere near the weight limits they list and I'd never trust my weight to them.
I don't know how old your kids are, but is there a way you could split up into 2 rooms? I've just plugged in options if you're not trying to put 5 all together in one room, and there are some accessible rooms available if you can manage splitting up.
If there are really no rooms on property for your dates and you can't manage otherwise could you either change dates or consider staying off-property? It does mean you'd need to figure out transportation, but may open up the possibilities for things you've never tried staying on-site. Any off-site hotel that provides transportation for non-disabled guests to WDW, would need to also provide it for you at the same 'cost' (or lack of cost). You could also rent a car (it's much cheaper if you look for car rentals away from the airport, even if some of the hotel-based rental locations are cheaper) and possibly explore some of the non-WDW attractions. Offsite just plain gives you more options of places to look at. I can't see every single hotel in the Orlando area having all their ADA rooms booked already.
