It's bad etiquette. I hate these kind of things where someone knows it's poor form enough to query it, but they're not really querying it, they're saying, I'm going to do this thing that's poor etiquette, let's find someone to agree with me, so my bad etiquette feels more validated.
To be unaware is one thing, but doing it knowingly is not cool on someone else's wedding day.
With the regards to the woman who has commented on lots of quite calm and factual posts accusing people of "losing their shit", don't exaggerate to make people sound unreasonable 
There are other people to consider. With regards to my aunt and all her clan turning up in buttonholes, my thoughts were, "nice one, I've planned everything to match and now these don't" and didn't really think anymore of it. What did happen though, is that my other aunt presumed I'd ordered the bloody things, and deliberately excluded just her and her immediate family.
"Buttonhole Aunt" meant no harm. But by deciding she would accessorise/dress her whole family in something that only the bridal party should have had, she didn't piss me off, she caused ill feelings for my other aunt.
Something as obvious as dressing your child in a dress sold as a "bridesmaids dress" on the website, to go to a wedding where they have not been chosen as a bridesmaid or flower girl?? Just no. Hopefully if you insist on putting her in it, you won't actually take the shine from the actual little bridesmaids, but either way it looks like you weren't bothered if you did. You might piss off people you don't even know because of wedding politics because their DC isn't in the bridal party and they assume yours is because of how you've dressed her. It's such bad etiquette.