Why is it up to the other parents to guess you would even consider letting their 10yo watch a 15? Until my DD1 was shown an overage film it hadn't crossed my mind that other parents would do it.
When you're trusting me with your child, you're trusting my judgement with his safety. Saying that I will use my judgment does not mean I'll have your 10 year old watch porn, it just means that if I think a movie is fairly innocuous I'll probably not ring you and ask. Besides, if the kids are watching something, it's going to be something that I think is also appropriate for my child (who'd be the same age as yours).
Because if we start doing things this way, we can go to fairly absurd extreme. Should I also check with you first whether he's allowed to have seconds for dinner, and whether having dinner at 19h30 is okay, and whether he's allowed to read this or that book, and whether he can play in the garden without supervision, aand whether he can play video games... That could be a long checklist !
It makes more sense for the parent who has particular concerns to proactively bring them up. For instance, I'd fully expect you to tell me without me asking, if your child has an allergy.
And, come on, movie ratings are often ridiculously stringent. Probably because people who submit them don't want to face a lawsuit if a kid has a nightmare after watching (considering the size of the audience it's pretty likely that a few kids will, even if the movie is perfectly age-appropriate).
Plus kids are not that fragile. And if you see them starting to be upset there's always time to stop the movie then.