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My Daughter is after a new bike as she's grown out of her Frog bike - it's got be ok for gravelly cycle trails, commute to school and some junior triathlons, she tried out a 47cm Whyte Devon which she loved but its about £500 more than I'd like to spend...
Any obvious options? happy to build up from a frame as we've a reasonable parts bin at home
You didn't mention what age / height, but my 9yo son (and a lot of others that he races against) ride Scatto frames, which have superb geometry for kids race bikes:

Short wheelbase, low standover but 700c wheels (very important for CX!) with loads of clearance for mud. Carbon fork and nice and light. Really flexible bike - one day it'll be thick with mud, the next racing on the track, the week after doing full day touring rides with his dad.

I picked this one up on eBay, but kidsracing and Paul Milnes in Bradford also sell them.
Thanks @Daern she's a bit taller/older than that (but handy info for my sons next bike!)
I think she's about 152cm/5ft so at the small women's rather than tall children's sizing? the Whyte was a 47cm frame
I spotted these framesets the other day, seem pretty good if you fancy putting one together yourself.
https://www.evanscycles.com/hoy-shizuoka-004-frameset-EV282214
https://www.evanscycles.com/hoy-alto-irpavi-002-2016-road-bike-frameset-EV271763
https://www.evanscycles.com/hoy-alto-irpavi-004-2016-road-bike-frameset-EV271764
You can get the Gensis Croix De Fer in XS.
Not sure what the seat tube measurement is though.
My other half bought hers for about £600. It looks great in the green/black/gold colours.