Children do the darndest things, don't they? Yuk! They sound very creative!
I'd go for comet cleanser or fantastic or 409 and stuff like that. Comet is abrasive and has bleach in it which is good for the germs. A scrub brush or old rough washcloth or scrubby sponge will be needed. You can also try steam (haul a boiling teakettle in there and keep the kids out of the way). Steam even gets wallpaper off!
Good luck! My daughter did this all over her crib bars, mattress, and the wall over the crib once during a supposed nap. She's a doctor now - very curious mind! She also made concoctions in my blender while I was busy elsewhere, sometimes forgetting to put the lid on. You can only imagine the mess! Of course, the poop was the most disgusting!