When my mom was a kindergarten teacher at an inner city school in L.A., she used to tell me similar stoires like yours. Except that the kids returning to thank her were teenagers who now towered above her.
Retaining applies to water, weight, employees, a lawyer, possession, and even facts, with varying degrees of short term benefits.
Teaching a child that they have the potential to succeed through love and respect is the foundation for an ever evolving life with limitless possibilities.
Always remember to be grateful you are doing the important one.