For young children, it's not necessary to teach technique. Painting at this stage is about unleashing your child's imagination.
How do you teach a young child to paint? The emphatic answer is don't.
When young children paint, they express how they feel and do not necessarily depict particular objects. What is important is the act of painting, not the finished product. Provide children with plenty of materials and opportunities to paint.
When your child shows you her work and it looks like random splotches, refrain from asking, What is it? or What does it mean? Instead simply allow your child to talk about the piece using her own words. Allow your child’s imagination free rein and don't steer her.
To get started, here are a few practical tips:
The Art Lesson by Tomie dePaola (Putnam Juvenile, 1997)