They haven't "subcontracted" anything to child miners, as nicely inflammatory as that sounds. They bought batteries, simple as that.
And the battery manufacturer didn't subcontract to child miners - They bought individual mass-produced cells and wired them into the desired form factor and electrical characteristics.
And the battery cell manufacturer didn't subcontract to child miner - They bought the various electrolytes and pre-made membranes that get wrapped up and turned into individual battery cells.
And the electrolyte manufacturers didn't subcontract to child miners - They bought simple precursor chemicals that they use as feedstock in producing highly specialized battery electrolytes.
And the precursor chemical manufacturers - Think names like DOW, DuPont, BASF, Exxon, Eastman, etc - didn't subcontract to child miners - They bought cobalt metal on the open commodities market and turned it into convenient, commonly used reagents that have a million and one downstream applications.
Now - The cobalt refiners, they might have bought directly from the mining companies that in turn use child labor. Of course, they no doubt buy from a huge pool of mostly-legitimate miners and don't have the resources to police every hole in the ground that sends them the occasional barrel of crushed ore.
But yeah, let's blame Samsung here for one small portion of a looong supply chain over which they have little control beyond their immediate vendors.