Golly, I would have so many responses to that as a justification…

• Do kids really still get allowances these days? I never did, and I don’t think anyone I know from the same generation ever did.
• It goes against the whole fundamental capitalist idea that equality of outcome doesn’t matter. If I suggested that poor people should be given higher wages because they have less money already, they’d call me a crazy commie but somehow in this one context, having money from another source means you should be paid less?
• It doesn’t even work on its own terms. If kids should be paid less because they get allowances then adults should be paid less because they obviously have way more money than kids! Even if you subtract their expenses