At Color with Leo, a young Leonardo da Vinci (Leo) guides children through interactive games and activities as they learn some basic principles of art.
There are three sections: colouring pages, games and art studio. There are also some ads, but I found them non-intrusive, and well chosen ie mostly "educational" products for kids.
Art Studio has some cute online art activities, from general drawing pads, to specific how to draw. An example is Portrait Fun which is an easy drag and drop way for kids to build a basic head. The activities don't allow for much creativity on the child's part, but that makes them particularly well-suited to young kids. The directions are clear, and the activities work well.
Games include very simple puzzles based on real works of art. Some of the games involve using the keyboard arrows and space bar, which The Book Chook found a little tricky. But a game like "Does Not Belong" is simple enough, and good practice in visual discrimination.
Best of all I liked the page for parents and educators, which provides free downloadable lessons and art education material in the form of pdfs. This might be useful to home-schoolers, and other parents looking for creative ideas for their kids. I especially like the fact that some activities make a great follow up to literature you've shared with your kids like The Very Hungry Caterpillar.
A web space like this can contribute to your child's developing visual literacy skills by encouraging him to become aware of differences in shape, size, colour, pattern etc. I love the idea of kids having fun while being exposed to new learning opportunities.