Shimano 8-speed cassettes fit the same freehub. I used to run 8-speed Sora on a 10-speed Ultegra hub, worked like a charm. With 11-speed compatible wheels, I imagine you'd need the 1mm spacer to make it a "10-speed" wheel. Did that on my Trek 1.2 before I upgraded the components, and on my gf's Dulce before she got a better bike.

But... Why deep carbon for a kid? Plenty of great, light alu wheels (especially the pre-11sp era) being sold for cheap that will be a significant upgrade from the stock wheels.
Unless, of course, "kid" refers to a 16+ Junior racer in which case he might grumble about a Sora-equipped Dulce soon enough. If it's a kid kid, maybe something less flashy (and easier to control in crosswinds) might be better? I'm a fully-grown, though lightweight, adult and I wouldn't road-race on a wheel deeper than 50mm. I'd rather not get blown around.

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ZONE3 - We Last Longer

You can leave the freehub on and just use a spacer for it to accommodate the eighth speed cassette.

Why not upgrade his component group before upgrading his wheels? Go to Ultegra and you'll save just as much weight as new wheels.

Thanks everybody! He's at the age where he thinks carbon wheels look super cool and doesn't know anything about much else about the bike. I could put Dura Ace electronic shifting on it and paint the frame with moon dust and he wouldn't care at all. But with the wheels, he'd go out and train more and harder because he's so into them, and that would actually be the thing that makes him faster. "The fastest bike is the bike you love and train on" kinda thing applies here. Currently I put my old Zipp 404 on the front for his races and he goes ape shit over how awesome it looks. Just looking to round out the package. Thanks again!

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