I did this (Sistine Chapel experience) with first graders. They used
crayons on 12" x 18" white paper. They taped the paper to the bottom of
their desks. I was on a cart back then - about seven years ago. They loved
the experience. I wasn't much interested in the "product" with that lesson
but mainly with the "process." Still, their pictures turned out so cute. I
think I had them paint things they would see in the sky. They could include
angels if they wanted to or just stick to stars, planets, planes, rockets,
etc. The crayon worked well because I didn't want paint to drip in their
eyes or to get on the bottom of the desks.
They untaped the pictures and put them on top of their desks to go over the
crayon again. Then they painted over everything with watery black paint for
a night sky effect. They had lots of fun.