You didn't tell us what type of paint you are using? In elementary school,
we covered a crayon resist of the deep sea with watercolors, covering wet on
wet, dry brush, sponge painting, salt on the paint, and graduated colors of
watercolor. You might try using white tempera also to show them the difference
between opaque and transparent colors. Scumpling colors and using a cotton rag
to pick up colors can also help.
I went to a workshop and we studied Oriental Brush Painting with india ink
and watercolors to draw orchids. We used bamboo brushes which cost about $2.00
a piece from Chaselle Art Supply Catalog. We practiced on cheap newsprint
paper. This might be an extension for the watercolor lesson.