Hi Debbie, why don't you think to upgrade the original frame of the Legacy (hard to find IMHO) to the HP (High Performance) type?
These are the spec. from Prism:
Legacy HP frameset upgrade
Trim 4 oz. off your Legacy
Fly in winds from 1 - 18 mph with this easy to install upgraded frameset.
Reduces kite weight to 10.2 ounces.
Sky Shark P100 Leading Edges
Sky Shark P300 Lower Spreaders
Sky Shark P100 Upper Spreader
Sky Shark P300 Spine
You'll need also some some interal ferrules for LE and LS and a couple of nocks, the Zephyr or E3 one will work perfectly, some adesive shrink tube (9mm diameter) as APAs stoppers and a center T. Just in case you may want to get two 5PT as LS to further increase performance.
Have you got bridles?
Measures are the same as the original frame and I'm sure that Steve will help you on all you need, obviosly you'll need new LE and LS APAs.
With this upgrade you'll increase the wind range and flying ability.
Hope this helps to get a nice Christmas gift for your son