So I will not sin against thee" ...
It's in the Bible ... look it up (Psalm 119:11)
We can't live the kind of lives God wants us to live if we don't know what he wants ... we won't know God as well as we might if we don't know his Word ...
Living without knowledge (heart knowledge) of what the Bible tells us, and expecting to be able to show God to the world is like showing up to star in a play with no rehearsal or no script, or going to a foreign country with no knowledge of the language or its customs, or showing up for the first day of school and not know where the building is, who your teachers are or anything about the rules.
Ya think that might help children understand some things about why we commit scripture to heart?