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The jump in handling (especially in the air) between a 24" hardtail and 27.5' full suspension is going to be a lot. For what it's worth I've had the same dilema for my just turned 11 5ft giraffe :o) I decided to go systematically up through the wheel sizes and so at 10 he was riding a 26' Chromag Minor Threat (yeah he's very lucky... but he puts it through it's paces in Whistler, Squamish, Sun Peaks etc) He's just moving up to a 27.5 Transition Scout this year. I wouldn't focus too much on the actual wheel size but...

Commencal Clash XS is an option. It's a 27.5/26 mullet designed for tweens. It's fairly close to an adult-sized bike but not quite. My friend's son has one and it's awesome.

Long time reader and first time poster. I don't have kids, but do work in the industry and love seeing young kids falling in love with mountain bikes. A couple thoughts from sizing bikes for a long time in bike shops. Kids tend to have much longer limbs than a full grown adult at the same size. What this leads to is being able to "stretch the rider or shrink" the size charts that most brands post. Example below with the 27.5 Giant Trance X. Super rad shredder bike, especially for a lighter rider, but the size chart suggestion starts...