What were Israelites to tell their children who asked about the twelve memorial stones?

Answer: The Jordan stopped before the ark

Joshua said the stones would remind them that "the waters of the Jordan were cut off before the ark of the covenant of the LORD."

“you are to tell them, ‘The waters of the Jordan were cut off before the ark of the covenant of the LORD. When it crossed the Jordan, the waters were cut off.’ Therefore these stones will be a memorial to the Israelites forever.””— Joshua 4:7 (BSB)