My bathroom has white tiles, so when my kids  (5yrs & 2 yrs) want to paint, I either put them naked in the shower or bath with some poster paint and a paintbrush. 

They have so much fun painting the walls, themselves and each other.  When they are done, I just turn on the shower, wash down the walls and the little people and all the mess is gone in a flash.  They absolutely love it!

If you have the space available in your bathroom, you can save a lot of time off cleaning it by creating a bathroom cleaning kit that you store in the room. Fill a cleaning caddy with everything you need to clean - not having to search for these things will make cleaning more efficient.