The issue I found was with technique - getting kids to sponge their paint lightly to create a cloud-like appearance, and to layer their colours. I also had a few instances of dragging the paint. In retrospect this activity proved a bit frustrating for the younger students and would have been better suited for an older age group, who would have more patience and therefore be more successful with the sponge technique.
The Students' Artworks
You can see the dragging of paint on the right side of this student's artwork.
In the artwork above you can see how this student had difficulty with layering different paint colours.
What art lessons or activities have you tried where students struggled with a technique?