I found this thread quite useful,  just looking at building a bike for my 11 year old.  In the end I found this http://www.rczbikeshop.com/default/rcz-2015-frame-junior-race-27-5-disc-matt-black-red-size-14.html#

Just the size I was looking for and not too heavy either. A full carbon can wait a couple of years till he grows out of this.

Detail from the site for anyone interested;

For 27.5" fork and wheels
Fork travel: 80 to 105 mm
Construction: AL7005, triple butted, hydroformed tubes
The rear bent offers more comfort
The top tube/rear stay intersection is designed to ensure the optimum weight distribution
The Tapered headtube offers more reliability and rigidity

Specifications:
Travel: 80-105mm
Seatpost diameter: 27.2mm,
Front derailleur: 31.8mm
Headset compatible: Semi-integrated 1.5>1 1/8 Tapered (Ø44/56mm)
Bottom bracket: BSC 68mm
Brake: Disc IS
Rear axle: 9x135mm

White with Red stickers

 Derailleur hangers are delivered with the frame, disassembled and packed in the parcel.

5 years warranty

GEOMETRY:

Seat tube   Top tube effective   Chainstay   Headtube   Seat angle   Head angle
355mm   528mm   430mm   120mm   73°   68°

Cheers

Jmh