Well you could get the 5th and 6th grade books and go through the 5th grade book and for every topic in the 5th grade book...say it is adding fractions do both the 5th and 6th grade work, or if it is the topic of probability do both 5th and 6th grade work. Don't do every practice problem presented by the books.

If your child can do 3 problems of every type discussed in that lesson...move on.

...reading is pleasure, not just something teachers make you do in school.~B. Cleary