While what Reid did was lenient, I gotta agree that it was rather fitting given how he still basically saw the kid as, well, just a kid. You throw a kid in juvie at worst, they're still too stupid to understand the gravity of their actions.
Not only that, he disabled the kid's proclaimed source of power (the magic tool) and explained to both the kid and other children in attendance the real depth of what the kid attempted (that dabbling seriously into magic is legitimately risking not just one's own life but also others'). I think he handled it rather well, all things considered.
That said, the kid was a stubborn classist anyway, so per the manga tradition I suppose it's also a setup for a stupid vengeful noble subplot for later on.