We recommend that you start with Life of Fred: Apples if your child does not know the addition tables cold. If your child knows the addition tables but not the multiplication tables, we recommend starting with Life of Fred: Farming. If your child knows the addition and multiplication tables cold and is in at the least fifth grade, then start with Life of Fred: Fractions.
The author does not recommend Fractions for students younger than age 10 or 5th grade. 5th graders and some 6th graders should start with the Intermediate series (Kidneys, Liver, Mineshaft) before starting with Fractions. They'll even benefit from the last two books in the Elementary series as well (Ice Cream and Jelly Beans). After they know your addition and multiplication tables, they are ready for Life of Fred: Fractions. Students will go far beyond elementary knowledge of fractions! Even if they already know how to divide up a pie so that they get the biggest "fraction," this is absolutely the right place to start.
NOTE: If your student has finished all of the Elementary books but is not old enough for 5th Grade, we recommend that you go through the Elementary books again. Like any good book, you learn more the second (or third) time through. You don't need to hold your student back, but your child will not be behind if you wait until they are 10 before tackling the Pre-Algebra series.
It is also recommended that you have younger students (1st or 2nd grade) go through the first few books with you before doing them again on their own. This will give them necessary review and give them time to prepare for Life of Fred: Fractions.