ok. so our MC figures out someone is stealing the stones.
sets up trap to figure out who it is with bad stones.
discovers the culprit
investigate culprit.- realize be useful and the whole children thing
trap already in motion so goes to deal with it
saves merchant for kids and now go way to handle distribution.
now powerful crime'lord type merchant in his debt...
that better then letting the king make fried pork. plus MC sees the whole trade web, not just the strands related to the demon king army. for they all connect, thats way commerce happens on that scale.