That's annoying, yeah. Sheesh.
This reminds me a lot of what happened at our last house. We put up that same kind of fence. A few years later, our neighbors put up, not a trampoline, but an addition the 2nd floor of which starts at about the level of our 6' fence and (because the house of course is level, and our fence follows the downhill contour of the ground) rises above our fence with each inch it extends outward. It was quite a thing when they put the floor on that addition and I first saw the workers standing on it – standing well above our 6' privacy fence. Oh, and this addition was also kicked over toward our fence by like 1 1/2 feet compared to the original house. It was utterly ridiculous, but totally legal. Our backyard was already, as I called it, a display case; but now it was to where I would feel 100% in display every time I stepped out there. It's most fortunate that my wife had already been clamoring for us to either add onto our house, or (as I'd prefer as an alternative) move. So we dodged a big bullet by moving.
So yeah. I can feel your pain there especially in how these kids will be bouncing up and down. This could be a big issue with your Dobes, who are probably excited by the movement, and the accompanying yipping and yapping by the kids. I'm sure they might even intentionally try to mess with your dogs. I'm sorry you're dealing with this. Honestly, I hate having neighbors within sight of me. They really are a big part of your life, like it or not – sort of roommates-light.
What would be the penalty if you put up a fence extension and were turned in and cited for it or whatever? Hopefully you'd just be told to take it down and not fined monetarily. I'd go as inexpensive as possible. This goes to the wayback machine, but I remember as a young kid in CA, our back fence had an extension on the top of it made of some rather unattractive green fiberglass or something.
You could explain as nicely as you can muster that you're putting up the extension because the dogs will be too excited/disturbed by the jumping. Hopefully the neighbors will not be spiteful enough to turn you in. Hopefully the kids will not be stupid enough to throw stuff over the fence or shoot water guns over or whatever.
I hate to say it, but I have seen a couple examples of kids' trampolines that receive pretty consistent use over the course of months and a few years. I wouldn't count on your neighbors' trampoline use fading, but I hope it does!