Get the children to cover a plain white A3/A4 sheet of paper with different coloured oil pastels. Make sure that there is NO white on the paper. Then they need to cover the paper on the same side with black/dark brown paint.
When and only when it has dried completely, get some scissors and tell them to scrape out a picture (make sure they don’t go through the paper).
This activity is great if you want it to look like fireworks.