If I owned my own house then I'd probably go loopy and paint one of the outside walls white and let them have at it, but we rent, and so sanity prevails, well, most of the time.
Ideally having a couple of large artist's canvases would be the solution, but well the size that I would want them to be would cost a bit too much for my liking. Yesterday I figured we have this princess play tent thing, why not just cover one side of it in what was left of a tin of white latex primer and see if that worked. It really needs another coat to totally cover princess Aurora, but that'll have to wait until I have more primer.
My first thought was to paint the whole thing so that we had four canvases that four kids could stand around and paint, but I only had enough of the primer left to do one side.
It was windy that afternoon, so I folded it up and clamped it onto our fence for the kids to paint on. We painted lots of other stuff yesterday and today, but I'll post about that later or this post will end up longer than I am tall.
The paint we use is just regular kiddy paint, but mixed with a dash of dish soap, so that it washes out of stuff easier. Adding a little bit of water also makes it go further.
It hosed off easily, and the kids could paint it again. It worked so well that I'm wondering if I should cut the four sides of the tent apart and coat each one with primer, so we have four hunormous canvases that can be hosed down and used again. It ain't pretty, but it does the job.
If you want pretty then maybe you can find some large ugly paintings at a thrift store to slap primer over, or make your own from 2X4 and old bed sheets. We paint old bed sheets all the time, but usually flat on the floor, or put up like tents with string and pvc pipe poles.
A whopping big bit of plexiglass would be tremendous fun to paint on too, like the one they have at the Bay Area Discovery Museum in Sausalito. The kids can clean that with window squeegies. Aw man, now I want to buy plexiglass.
If you have an old tent that you are happy to just have the kids play with, then that would be a lot of fun! Imagine having a whole den to paint and then hose off and paint again another day!
Me and two of my mates spray painted an ancient two man tent in 1995 to go to the Glastonbury music festival. Yes, three of us slept in a two man tent, with an open tin of golden syrup we later found out (luckily it was a good weather year and we could all sleep under the stars for the rest of the festival, because that Syrup was not going to be cleaned up easily!)
The kids do end up making a hell of a mess painting, but between you and me, I really love finding a blob of dried on paint in a place that it doesn't belong. It says "someone had fun here".