Hi Siobhan,
flour is a good surface for this, but needs to be disposed off afterwards (food source in the classroom). I tend to use different sands that I get from B&Q or other suppliers. The sand can be dried (so if you get sand, allow about a week for it to dry enough to use) so that it flows easily and makes simple craters. If you use wet sand the balls just drop and make a hole exactly the size of the ball!
This is lovely if you look at some of the craters that have clear ejecta – like Tycho.
https://science.nasa.gov/resource/tycho-crater-on-the-moon-labeled/