They are a set of related problems. Whenever you work on subtraction, practice with related sets. This will help them discover how addition and subtraction are related. Even if they seem to get it right away, continue working on this and the related sets for a long time. You want this to become natural, and second nature.
Public schools cover so many math subjects in one year that the children forget everything.
This is why schools re-teach every subject every year. If you just drill in a few subjects, they will learn the subjects perfectly, and will need little review next year. If your children can write, you can start teaching them to write out math problems, or at least write the answers to problems you have written out. Try writing the problem on a wipe-off board, and letting your child answer there. They should be allowed to use the toys for a long time before you begin memorization.