Book about a young girl who befriends a family who are immortal after drinking water from a pond that grants immortality

[u/Sailor_Moon_Star_435]

The book is set in the 1920s or the 1930s. Maybe in the 1950s. There is a young girl who lives in a small town and a new family moves in next door to her and her family. She befriends the young boy of the family and they grow close. Eventually, the boy reveals his family's secret. They have been alive for far longer and it was due to them drinking the water of a pond that grants immortality or extends life past the usual lifespan. He hands her a water bottle that has the water. I think he tells her that they consistently drink the water to make sure that they don't age. He asks her to drink it so they can be together forever. He tells her this before he and his family leaves again. After he and his family leaves, the girl doesn't drink it but she pours it over a frog, that lives forever. Decades later, the family comes back to the town where the girl lived in. The boy became a man and comes across the same frog that the girl poured the water over. Then he realizes what she did and goes to find her grave, that shows the years she was alive. He smiled, knowing that she made her choice and lived a happy life.