These keep popping up all over the place, some take them and place them else where others keep them I leave them for others to see each is OK according to the facebook group who do them ' love rocks in lincs' ian
There is a school children's group, who go for walks, find a rock, do a painting on the rock/pebble, then hide it. When a child find the rock, they replace it with one of their own, take it back to school and get a mark. This is a way of first getting them out of doors, albeit to local parks, and finding about nature at the same time. This is an after school or weekend activity and does involved the parents hopefully encouraging them as well.
I've got several paintable rocks lying about in the garden... if I did something with a Christmas theme on them, maybe that might get something similar going in our woods. There is a school nearby that often does walks and outdoor activities there.