Maybe they have decided to rent until the youngest child finishes high school, and then they will be ready for the empty nest phase and will re-evaluate. Who knows...
My husband and I were renters in the same house for 8 years. The owners loved us - we took nice care of the place and the yard and even did some upgrades (with the owners' blessings).
Look for document templates that you can use as a rental contract. Put some kind of clause in it if you decide to put the house on the market again that gives them a fair amount of time to relocate, if the house sells.