Oil pastels are a type of art material that look like colorful crayons. They use oil to help the colors blend easily, making them great for drawing and painting. Full definition
Once the paint is dry, you can use oil pastels on top of paint to add thick, heavy outlines or to add accents on parts of your painting. (royalacademy.org.uk)
We used oil pastels in this activity but you could use colored pencils or colored markers. (artforkidshub.com)
If you and your kids already have a set of oil pastels, I'm sure you've realized they can be messy. (artforkidshub.com)