Give your children opportunities to express themselves artistically - even before they are able to make something that is identifiable! A friend of mine has a beautiful piece of art, hanging in her home, matted and framed, that is colorful and modern-art looking - and done by her then-three-year-old. A creative mix of colors or textures, when matted out in basic black or a color of your choice can be a thing of beauty!
I like to introduce children to art one medium at a time - and let them fully experience it: what it feels like, looks like, what they can make it do ... with some very cool results!
Crayons are usually one of the first art tools that a child comes in contact with - they will make "scribble" pictures, loops, dashes, color one color on top of another ... all methods of experimenting with the medium. And ... all good small motor activities, pre-writing practice, color mixing. Use your child's art to build instruction - ask them what they see, what they want this to say (because some of that "scribble" is "writing"!), what colors they see ... open-ended questions that will encourage language development as well! Just because you don't see a picture of a person or a truck doesn't mean that the picture has no value - it is just the beginning of the process that will lead, eventually, to more recognizable shapes and objects.