In case anyone is interested, my coats arrived. One for me and one for DD. I'm 5'4 and just over 8 stone and the small coat was enormous on me; the arms were too long and the waist was huge. I measured myself before buying it and it said that it was the coat for me. I tried on my DD's XS petite one and that fitted perfectly. I guess the coats are bigger to leave space for lots of layers underneath, but wearing a massive coat isn't a very flattering look.
The biggest kids size is 9-10, which should be too small for my 10 year old, but I looked at the waist size and it's the same as for the XS one (which was huge on her).
That said, the coat is beautiful and I have had loads of compliments on it. People don't usually compliment me on my coat,s but it really is beautiful. If you're thinking of ordering, you might want to get a size smaller than they recommend.