Hands down, Diamond Razor Edge.
Without a doubt this bow is the best youth bow for a growing child made right now. It is adjustable from 30 - 60 pounds with out needing to replace limbs and draw length is adjustable from 19 - 29 inches with out the need for a bow press.
It retails for $329.99 as a complete package. You will need to add arrows and a release. It is also available as just a bow if you want to buy your own accessories.
At 29" and 60 pounds the razor edge will fling a 350 grain arrow at 291 fps. It will shoot an IBO spec arrow at 310 fps. This is a smoking fast youth bow. One of the absolute fastest youth bows I have ever seen.
The Mathews mustang is definitely a nice bow but there is no adjustment what so ever in draw length. You must buy new cams every time you need to move to a longer draw length.