I'd invite the parents over one night, so they can hear how loud it is in your place.

I'd explain that the kids seem like great kids, but notwithstanding that you've mentioned it a few times, they're still doing it, and so you wanted them to hear how loud it is in your house - at a time when you're trying to get your kids to bed.

If someone showed me in that way how it's impacting them in their home, that would have quite an impact on me. And I would feel like it was respectful - they're not just asking me to take their word on it, they're introducing me into the conversation.

If you need to level up, I'd go to the council.

Your local council will have something on their webpage about noise levels - if you google the website and "decibels" I would imagine you'll find the page. If you're going to make a complaint, you'd probably need to take diary notes over some time, and you probably would be helped by downloading a decibel reading app onto your phone, and taking screenshots to show the decibel level and time, to back up your diary.

But be aware, they'll probably know it's you, and if you're not ok with that, then don't go that path.