If you're on good speaking terms with their parents, maybe you could have a friendly word about privacy/boundaries? If they're told as many times as necessary that it's polite to ask first before joining in with the neighbours, it might start to sink in.
Been up to the allotment this morning for some more weeds out/seeds in business (and yet more rhubarb...what to do with it this time?) DS is out of sorts today though, uncharacteristically grumpy and sparkle-free, not sure what's going on there.