I don't really like it for an 8 year old. I just don't think it's right for that age group I don't think anything that immodest about it, necessarily, since my kids might have bare shoulders on a summer dress, and my younger one might still run around shirtless; but that feels like bare shoulders for a different purpose--casual, for the purpose of being cool. This dress is clearly meant to be formal and has a design that will flatter or enhance an adult figure, which an 8 year old doesn't have. So the bare shoulders & back have a different meaning for adults in a formal setting, and it seems like just putting a child into a more grown up dress that makes them look more like miniature women like the other bridesmaids. I would prefer a dress that makes the junior bridesmaid look like a child, differentiated from the other bridesmaids, since there are limited opportunities to wear these kinds of fancy dresses, and childhood is so short. So I think it's more that I would be disappointed that they chose this kind of a dress. It can cover enough to be far from immodest, but would make her look more awkward than cute, IMO.
But who knows, maybe it could be altered to look really cute, and my daughter chose an Easter dress with wide straps, but that bared her shoulders when she was 9, almost 10. Then the next year a halter style dress, but she insisted on wearing a shrug with that, at least while indoors. Seeing it on a little girl, it's kind of cute, but I'm still not convinced. My kids have stomachs that kind of pooch out, and my 11 year old hates anything that has too much material around her chest like that, especially if it makes any part of her look "fat". But, you know, that's a completely different issues, and I hope other girls aren't as self conscious as mine is.