5th graders! what?, about 10 years old?...
I think it comes naturally. When they make landscapes, do they draw
hills which cross over? If so, then they are probably ready for the
sky to meet the ground. The base line is still probably pretty
evident.. I.e. trees which grow along the skyline or do not go beyond
the top of the hill.
You could probably teach painting technique. Pulling the brush rather
than pushing it back and forth. Consider getting them to fill in the
background prior to painting detail.Use a roller to make the sky then
paint detail over that when dry.