We've started reading the delightful book "Life of Fred: Apples"
from Dr. Stanley Schmidt's Elementary Series.
Each hilarious chapter is short (6 pages) and concludes with a handful of questions for the kids to do. It is really intended for Matthew's age and so some of the questions are simplistic for Bekah but then others are very appropriate ("Do you think a million is a big number?"). Amazingly enough both kids are entertained and starting to do simple algebra in a very natural way.