Catherine,
The cratering activity is such an open activity. I’d suggest providing a range of items to drop, including irregular shapes. This will let the children test if round craters only come from round objects.
The Curious Minds activity outline: https://www.sfi.ie/site-files/primary-science/media/pdfs/col/meteorites_activity.pdf
suggest this for 1st class and up and proposed just spherical objects.
We do this with children in workshops, and find that they need a certain initial ‘play’ with the materials to get a sense of what can be discovered with this set up. Once they have that familiarity they can carry out an extended inquiry into the factors that interest them. Very often fair testing gets ‘thrown out the window’ as they drop different objects from different heights!