You smile and say: "It's your wedding and I'm just happy to be in it! Are you giving me a choice of dresses and want my opinion or are you letting me know this is the dress I will be wearing" Be honest up to a point, you don't want to hurt the bride it's her wedding and you suck up and wear what she puts on you and pretend you are wearing your dream gown so you don't look like you are in pain walking down the aisle!
I have worn MUCH worse than that as a bridesmaid....BIG bow in the back, puffed out short sleeves etc. Really, that one isn't as bad as it could be hahaha.
If it were me, I would just smile and wear it. Its her wedding, these pictures are going to be in her album, she needs to be happy with how it turns out. If you really want to say something maybe tell her "I don't love it as much as you do, but I love you and I will wear whatever you want, I'd wear a burlap if you wanted lol" That way she knows you dont like it, but she also know you want to support her and you'll be fine wearing it. If she is open to changing it she can then ask you for your opinion. She may just really want that dress on her bridesmaids though
I have been in a similar situation when I was in my cousin's wedding. She had her 2 sisters, my sister and I in her wedding. She tried to be considerate and pick a dress that was a decent price and would manage to flatter all body types. (not really). The color she chose was awful. Funny thing was I don't remember anything in the wedding besides us girls with horrible fushia color. My sister and my cousin are the best of friends. They do everything together. She even had my cousin as her maid of honor at her wedding. In this case she did not speak up as it was my cousin's wedding and her choice as to what she would like her brides maids to look like.
See that's what's bothering me too. I don't think there are many things that will look right on an adult and a child. I'd be happy with something plain in the same colour.
We all have the dreaded brides maid pics check this one out.. this was Aug 1989
There were a few people in the wedding party who were ummm we will say fluffy... I was a skinny 15 yr old I was a size 8 .. but because if the ruffle that was supposed to "hide" the baby bump on 2 of the brides maids and the behind on the other I got stuck with a frump dress... this was my then BF