And it shall be, when your children say to you, 'What do you mean by this service? (NKJV)

This verse refers to the Passover ceremony that the Israelites should conduct every year at the same time. The first ceremony was to mark their doors to avoid death by the angel. After that, the ceremony would be to memorialize that time.

The Lord knew that children would ask. This was a hands-on way for the parents to teach the children about what the Lord had done for them. Children get bored with lectures and preaching, they need hands-on activities to create muscle memory.