Go light Everybody's list should include a helmet and motorcycle goggles. Also, it doesn't have to be riding gear, per se, but you also should outfit your kid with long sleeves, long pants, gloves and heavy boots.
Go medium When your kid gets up to speed, they really should have the extra protection of specific motorcycle boots. The shin protection, alone, is vastly superior to even the sturdiest pair of work boots. And although riding gloves may not provide significant additional protection vs. leather gloves, for example, they are far more comfortable.
Go heavy If your kid decides to try racing or even practice days on the motocross track, it's time to get serious about riding gear. At the least, you'll want to add a proper jersey with built-in padding, kneepads, elbow pads, riding pants and a chest protector to the gearbag.
GO LIGHT
GO MEDIUM
GO HEAVY