Prioritize and Plan. Sit down with your pastor and teachers and come up with a list. Have a list. It is a waste of time to teach Psalm 23 to a group of first graders and never refresh their memories in the second grade, fourth grade and 6th grade. Have a plan.
Memorization is a multi-level process.
There are times when creating just a familiarity with a verse, instead of a memory, is worthwhile to your lesson objectives. It may depend on the verse or text being memorized. If it is the Lord's Prayer, they should know it word for word. If it is the second commandment against idols, the general gist may do. You may also want to make some choices about the memorizing of the verse and the memorizing of the Book and Chapter.