@woodyLdn
We have a v large treehouse at the bottom of our garden. It was installed by previous owners and has temporary planning permission. Drop a note through their door, politely asking if a solution can be found? If they're not helpful, then the council is the next step.
The problem with that approach is that the only solution is to remove it.
Putting a side on it, or screening it somehow still isn't acceptable.
So the OP really should be upfront about the end result she wants to achieve.
Hopefully they are reasonable people and on seeing a photo/ viewing in person they will understand the issue.
If not, well the fact of the matter is that a structure of that height/size/proximity requires planning permission and unless they have it, it needs to go.
For those criticising the OP how would you feel if your neighbours put up a large structure on your boundary that breeched planning regulations, substantially impacted the enjoyment of your house/garden and devalued your house - would you just suck it up?
No and if you say otherwise you're being disingenuous.
The structure absolutely looks newly built and may not have even been there when viewings were done so the idea "it was there first" is moot - there's no reason to know that's the case and again regardless it's an illegal structure the timing of its implementation isn't relevant.
What's telling on loads of threads like this is in nearly all cases the structure/trampoline/play set is invariably positioned as far away from the parents home as possible presumably to protect their enjoyment of their garden with zero regard for that of their neighbours.
So no I don't feel sorry for the neighbours or the kids.
Planning regulations are there for a reason and if you break them don't whine about it.