I'm usually one for etiquette, but this is next level.
It's a 4YO in a party dress.
No right-minded bride will care. No guest will care. I can tell you right now no male guests will care, anyway! The only female guests who'll notice - let alone comment on it - seriously need to get out more.
I have to say, if a guest at my wedding asked me, I'd be mortified that they thought for even a second, that I'd be the type to mind about something as inconsequential as this.
I'm not saying don't ask - clearly that's the accepted position on this - but the outrage on this thread is quite something.