Am I being a kill joy?

If the tree is on the Linkat property, yes and for sure.

The property line needs to be nailed.

Getting an accurate survey could cost some serious money. So could a law suit for injuries. While i agree that firmly establishing the property line is a good idea, tearing down what was erected and posting the property would be a good start--and then see from there. Any time she sees those kids out there, she and her husband should warn them off the property.

Youve sort of already set up enough precedent for an "invitation to disaster" by passively allowing the kids to play there. I keep my proerty posted and the post signs are clearly worded and fit our states requirement for spacing EVEN THAT doesnt protect me against lawsuits brought by some smart ass attorney who will claim the attractive nusiance bullshit should some kid get busted up by falling into a creek from a rocky bank that, in my childhood, woulda been an ideal "divey".

"Post and chase", thats my motto. However, If someone merely wants to hike through, let em stop and I will grant a corridor of time and place for safe passgae across the land, OF COURSE ONLY AFTER, signing the usual disclaimers , hold harmless and liability waivers and covenants "not to sue'.

That this is a reasonable rational approach in this day and age is so sad.....

I'm not sure how the kids secured the wood to the tree, but I've pulled out nails with the head of a hammer, or crowbars. But it's been quite a while since I've tried to do it, not sure my joints would be happy.