Narp, I haven't suggested anywhere that the problem would be with a six year old upstaging the bride. I don't think anybody has.
But what I (and several other people have pointed out) is that it causes other problems. Mainly ones which involve other children who haven't been asked to be bridesmaids (but are a lot closer to the bride) or children who actually are bridesmaids who are upset when another little girl turns up and upstages them and makes something which was special for them into something not so special.
It's a particular problem because those froo froo frothy dresses are to a lot of people a bit old fashioned and tacky anyway. A lot of little bridesmaids wear what are essentially floaty summer dresses now and not yards of netting. So, yes, if a little girl turns up wearing one of them it sticks out like a sore thumb.
People are asked to be in a wedding party as a recognition of their close relationship with a bride or groom. It is special and will be something that they may only do once.
So somebody elseover dressing their kid to look like a member of the wedding party and steal the thunder of little kids who actually are in the wedding party is just fucking rude.
And while it doesn't make me think anything about the child, I do look at the parents and think 'You are so precious about your child you couldn't bear the fact that for one day they're not going to be in the spotlight so you've dressed them up to get attention'.
It's not saying that the bride is being upstaged to point out that dressing your child as a bridesmaid at a wedding where they are not a bridesmaid is rude, attention seeking and says that you don't give a shit about the feelings of the people involved in the wedding. They come second to your little princesses right to float round done up like a dogs dinner in something out of the early 00s.