My favorite way to get rid of the clouds/sky issue is to encourage them to
make the other objects big enough that they don't need much sky. Adding
sky, clouds, sun and v-birds is often a response to the need to use the
space (or a habit). I try to give them alternatives, which often leads to
more interesting designs.
I really think the blue band for sky is a developmental thing. For five
years, I took K-5 outside and showed them that the sky touched the ground.
It was fourth or fifth grade before they got it. The little ones couldn't
see it at all.