A child who begins to draw, or build, or make up songs in order to please an adult has already lost some of the courage to experiment and enthusiasm for creativity that is so difficult to hold on to as we grow up. Let your child have as much freedom as possible in the area of creative play. Older toddlers and preschoolers should be encouraged to learn and follow rules that will allow them free access to art materials as soon as possible. Make sure you don't overdo the rules to the point that art activities become more trouble than they're worth. You should expect a toddler to spend only a very few minutes at any activity and elaborate preparation and clean up requirements may end up taking more time than the activity itself.
Simplicity is the key to success in toddler art.
As children get older, you can expect increasing attention span. Remember that the energy spent in conforming to rules of neatness and order will be taken from the energy of exploration and originality. Every child needs to learn to conform in many ways to learn to recognize the rights of others and to be aware of safety rules. Without a framework of reasonable expectations, exploration and originality tend towards chaos. The over civilized child, however, sacrifices originality and exploration for the sake of approval.