Does Nathan's Hot Dogs (or any other chain) generally forbid selling kid's meals w/o kids being present?

My guess is that it really depends on the place. Perhaps the kids' meals aren't really kid-sized, and it's basically a discount for being younger than (insert minor age here). If you're that worried about this, I would recommend you do send an email. Calmly - I repeat, calmly - explain what happened and rather than ask 'what the heck is going on with your business' ask why, exactly, they have this policy.