Lots of great ways.
When I was student teaching, a friend of mine and I did this too...we did the fabric crayons on muslin and then she transferred them with an iron...just a note of caution...make sure not to iron directly onto anything...the crayon went onto her carpet!!! But, to me, that way seems easiest, or our local Joanns has had kits with fabric markers and blocks and you just have the kiddos draw directly, then sash.
Another caution...make sure the total size will be realistic. I did 2 hand print quilts (with fabric paint and muslin) a few years ago with my son's K class and with sashing or non handprint blocks, the things were huge. I just ended up seaming the blocks directly together so they would be reasonably sized.