Liquid Chalk Markers are easy to use. My three year old watched me do one of the markers to get it started so you can use it for one of my youtube videos I had to leave the room for a min and when I can back she had one of the ready to use.
You can use this one the little chalkboard labels.
You can Not use them one a painted door and get it without the paint coming off as well.
You can pick up your own pack at Amazon.com
You can also watch my how to video here on YouTube
Enter to win your own pack here