Some years ago, my Dad had to fell the tree that was our 'tree house' and 'tire swing'. It was a huge wrench for the family. Many, many happy days had been spent in and around that tree.
Now, my Dad, being a bit of a wood turner, saved the wood and made bowls and stuff from it. The pieces were given to each of us one Christmas. Into the bottom of each piece Dad burned the date the tree was taken down, and a brief note for each recipient. Maybe you could do likewise?