I think that's a nicely-balanced approach to art with kids. So many of the curricula and guides that I used/perused through as a teacher emphasized time periods, art styles, and other -isms. While I think these can be fascinating, I don't think any kid is really going to latch on to that stuff...it's just not as interesting as getting to see another cool painting! Like you said, get them exposed to some (discerning) material, see what whets their creative appetites, and work from there.