Tall 12 year old, broad shoulders, small waist, bit of a tummy but absolutely no bum!

[BreadAndJamForFrances]

And she really, really wants a bikini. I think maybe shorts style bottoms would suit her better, but as she has no bum and not much hip I think they may roll down. She is a 34 B cup size. Anyone got any ideas of places/styles I could try?
I don't want to spend more than £15 really, as she is still growing so fast and I just had to but her new summer clothes!
TIA

try going into somewhere like newlook / primark and get her to try on loads of different styles and see what suits her! this is what my mum used to do with me!

Something like think this would suit her:
www.boden.co.uk/en-GB/Teen-Girls-Swimwear/96010-WHT/Teen-Girls-China_White-Bandeau-Tankini.html

Tie type bottoms may work - as she could tie them in as tight as she need - also being able to buy the top and bottom separately may help as she could get the appropriate size for each part of her body:

www.hm.com/gb/product/99016?article=99016-A
www.hm.com/gb/product/99015?article=99015-A

I agree with Becca Bubbles - it may be worth going somewhere where you can try on different styles of bikini - and then at least when you know what style/size suits her it would be much easier to find one in your price range/that she likes.