Recognize Bible memorization accomplishments through verbal praise or awards.
Psychologists remind us that memorization is achieved with maximum efficiency only when there is some external reward given to the learner: praise, approval, certificate, trophy, prize.
Children are different. No one else can turn on your internal motivation button except you. We can't reach inside children and turn ON their motivation buttons.
It's out job to find whatever to get kids motivated to learn God's Word. There are some who we can just challenge them with the importance of hiding God's Word in their hearts. Others, this will not phase. Their internal motivation will be accelerated only through tangible rewards