Beth in the UK writes:
If your little darling has drawn on the walls with wax crayons, use baby wipes to take it off. The sooner the better of course but if you don’t discover it until weeks later because they’ve hidden it behind the sofa etc. it’ll still come off with a little persistence.
Be it ready mix or powdered, if you squirt a bit of washing up liquid in the mixture it will come off/out of most things.