There are innumerable ways to create codes. Codes have been used in every conflict our Folk have faced and there were whole military departments that were dedicated to cracking codes. Give children a challenge to create an “unbreakable” code. One will be amazed how many ways a child will be able to come up with codes.
A great way to get them started is studying hieroglyphs and Rune stones. Teach children that it took archeologists several years, even decades to even assign meaning to different symbols and strings of symbols. There is an ancient runestone recently discovered in Norway under a church floor. (http://www.aftenposten.no/english/local/article2033018.ece) With other famous runestones paving the way, archeologists were able to decipher its meaning. Have your children create a runestone code that others have to decipher!