From chapter thirteen of Deathly Hallows:
Halfway along the next corridor he emerged into a wide, open space where a dozen witches and wizards sat in rows at small desks not unlike school desks, though much more highly polished and free from graffiti. [...] They were all waving and twiddling their wands in unison, and squares of coloured paper were flying in every direction like little pink kites. After [...] a few more seconds he realised that what he was watching was the creation of pamphlets, that the paper squares were pages, which when assembled, folded and magicked into place, fell into neat stacks beside each witch or wizard.
Not a sweatshop per se, since the working conditions don't seem unreasonably harsh. But the same basic approach. Note in particular the absence of anything resembling an assembly line, and that the pamphlets are constructed one at a time rather than en masse.
(Of course the pamphlets in question did not appear to be magical in themselves, but it seems reasonable to suppose that mass-produced magical items are in general manufactured in the same way.)