How about creating a very simple database as a back-end for a mock social-networking site? You can use the kids' actual names in the User table, and maybe create another table called Subject, for their favorite subjects (Math, English, Science, etc.). They can then learn how to query for, say, all kids born in a particular year with Math as their favorite subject.
They can research Normalization. What would the effect of de-normalizing the User and Subject tables into just the User table?