No. 68 Easy Under Drawing

Pastel pencils are a wonderful option for drawing guide lines for an under-painting. Use them on a toned canvas. 

The pastel pencils have several advantages. You can choose a color that will not muddy your first layers of paint, the pencils make fairly precise lines, and they are easily erased with a damp paper towel. 
Kid Friendly
[From "151 Uncommon and Amazing Art Studio Secrets"]