I'm going to echo this. "Please" implies an optional action like when you're trying to be kind to a child. As an adult some people will need you to do things unequivocally. If you can't handle that then that's really unfortunate for you. Please, get over yourself and understand that sometimes business is business and some thing must be done. They aren't optional. Otherwise you may find yourself finding a new job.

Having said that, I think it is wise to try to keep things amicable and only fall back to more rigid language and requests, stated as requirements, only after kinder communication has failed.

It's not that it reads as optional to me, as that it's kinda hard to read without seeing the implied "you should have done this already, without being told".

The tone, trust and context are massively important in these kinds of communications.

I don't think I've ever used this phrasing in writing, maybe because I also hear something similar to what you're hearing when I read it to myself...

Why is everyone using "passive aggressive" these days when they really mean "irritated"? The parents in your example situation couldn't be really accused (even implicitly) of being aggressive and they are definitely not passive.

And there's nothing wrong with being irritated.

The phrase used to mean responses like "I wonder why some people act like moronic pieces of shit about others' seats". See how it's non-direct but aggressive at the same time?

It's passive aggressive because it's a passive statement that carries a strong threat of negative reaction if it isn't followed.

Something like "will you please stop kicking the seat" carries a strong threat relative to which other variant? It's more passive than which other variant? Or do you claim that it's a passive voice grammar-wise? (It isn't.)

Of course it depends on the tone, the context, and the request itself.

When I'm asking someone to do something, I almost always want to know if that request is a stupid thing to do right then. So, "Will you please" implies my intent. If you don't think it's the right thing, then let me know and I'll almost always be persuaded by the new information.

If the reason is, "I don't feel like it." that's one kind of conversation, but the more common response is, "I'm right in the middle of xyz, can we do this tomorrow?", or even "I'm not sure we should do that at all because, x, y and z."

The answer is almost always, "Of course, thanks for letting me know."

All that said, a "Will you please" when you really mean, "Do this shit right now, or else!" is one of the most infuriating things people do and I've left an otherwise great relationship in large part because that nonsense was constant.