It would be great to break the ages down further, as you say, @Phil-and-Lisa1 , since I am in a similar situation. No disgruntled neighbour, but a treehouse that is unsafe for two-year-olds and yet a paradise for ten-year-olds.
For now, I go the extreme route of ticking 'not suitable for children 0-12'. It has not done any harm. Families still ask, even families with toddlers still ask, and I get to vet them. I have Instant Book, so if someone insists on bringing that toddler, I can cancel without penalty. I have never had to. They always understand when I explain/exaggerate the dangers: height of the platform, darkness of the woods, deepness of the river, hungriness of the coyotes...
Sounds like your neighbour is pretty scary, so use that... I'm kidding.
Tick 'not suitable for 0-12', and then when someone wants to bring an under-8, you can say no. I know you think they won't find you, but I suspect not everyone uses the filters, since they do find me.