Where can I get dresses like these?....

An 11year old in a halter neck pencil dress? I think you are going to need to wait until nearer the time to see what size and shape these girls are, but as a Mum I would be concerned about dressing up an 11 year old (thinking of Christina Ricci in Cher's 'It's in his kiss' video) without them looking as if they are in fancy dress rather than 'dressing fancy'. 11YO tend to have a tum and little/no boobs so pencil may look odd/inappropriate.
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I love the VoH dresses. I really really do. And I think they look lovely on most women as there is something to accommodate all shapes.

But I do not think you should be looking for the little girls' dresses just yet. They will need something VERY different, and if you can find it now, you may well not be able to find it or something similar in 2 or 3 years' time, which will be even more disheartening.

If I were you I would concentrate on things like your colour scheme, flowers, decor etc. Nowt wrong with being organsised, but you have time on your side, and changing bodies of girls, and therefore are much better to look much nearer the time. How about googling celeb Old Hollywood weddings of the 40's, 50's and 60's and seeing what the flower girls/bridesmaids were wearing then.....it would be much more "age appropriate" without being out of your theme/style.

Don't get the dresses yet. My bridesmaid is 12. We waited until August to get her dress, for a December wedding, thinking we'd left it late enough that all would be fine. It's now too small and I have to shell out for another dress. You just never know with kids. Leave it as late as possible.

I'd put both the children in circle dresses and get them made by a dress maker closer to the time. They'll be fairy cheap to make and you can get them exactly to fit them.