Since our last experience with our homemade chalk didn't work so well, we decided to try it again using a different (and simpler) method :) 1st we crushed O's wet chalk using the mortar and pestle. Then we added a bit of water and stirred.
Viola! Chalk paint!
This paint worked much better on O's chalkboard, and we had lots of fun! (aside from the fact that I was attacked by fire ants in the middle of the process! I'm just glad that they got me and not O!)