Give your young artist a good quality piece of paper and a choice of colors that all go together- all blues and greens, or all reds and oranges... Tell them too fill up all the white space with color. Sometimes I use painter's tape (it comes off easily) to make a border and when it is peeled off, the painted space really pops out. When the paint is dry, hand them a paint pen and let them draw whatever they want. This is one that Harry did when he was almost 4.
Framed, it looks like this (minus the weird distortion)
Once, I gave him a canvas and he made something similar for a gift for Nana.
Ava made the following triptych for Jonathan for his birthday this year. She is really into drawing these simple people with black line, so this project also worked well for her. A solid acrylic color on 3 blank canvases, then a paint pen for the people. Sweet.