I'm 30 and I have what your daughter has. In fact, she's the first person I've heard of that has this! It is really really rare - and I have searched the internet for years to try and find others like me.
I was born with a hemangioma/strawberry nevus on my left breast that was bright red in colour, and it grew as my breast developed. It covers the whole of my left breast including the nipple.
My parents were given very little support (I think because the doctors didn't really know much about it) and were told consistently that it would just "go away on its own" in a couple of years after it first appeared. Then they were told it would go away by the time I was 8, then by 12....and it didn't disappear - it just got bigger as my breast got bigger. It also used to weep a lot when I was younger so they had to stitch it up - which caused a lot of scarring.
When I reached the age of 16, laser surgery was quite new and they decided that they could use this to take away the redness (it was always very obvious when wearing a swimming costume or low cut top). It was painful (like being snapped with a rubber band over and over again!) and caused the birthmark to go black each time they lasered it - but eventually it made the birthmark go to a normal skin colour. The extra layer of thin skin, and scars from the weeping, are still there though.
But my main issue was still there - the breast hadn't grown properly (I had a similar size issue to your daughter), it was a slightly odd shape and not full and round, and the nipple is a lot smaller than my normal breast.
I live in the UK so the NHS (National Health Service) gave me a free implant on this breast - this is because I went to the doctor and explained the psychological issues it had caused me (not feeling like a real woman/self image problems/not wanting to take my bra off in front of boys). After the implant I didn't have to stuff my bra anymore and the breast was now full and round. Unfortunately over time I still felt lop-sided and uneven, as the fake breast was hard and pert, and the normal breast dropped a lot lower.
4 years ago I decided to have breast implants done on both breasts and the old one taken out.
Even though I'm still not fully happy with my breasts and the birthmark and scarring is still there - it's a LOT better than it was.
I wish there was more help out there though. I'd love to know if I can have the birthmark removed but I think it would cause too much damage...
Anyway I hope this helps you
Good luck with everything