The building in the children's playpark at Aline Community Woodland was vandalised sometime in the last week.
Walls inside the building were damaged, possibly by being kicked in.
Photos on the Aline Community Woodland Facebook page show the extent of the damage.
Said an admin: "Why someone would enter a facility in a children's playpark and kick the walls in defeats me. This is a public amenity created through the hard work of a small voluntary group for the benefit of children and their parents. If you saw anyone acting suspiciously please let us know so that we can track down the culprits."
Stornoway Police say they are yet to receive a report of the vandalism, and www.welovestornoway.com have emailed the Woodland for comment, without reply.