Some assert that free is only an adjective and not an adverb. Therefore, 'kids eat free' causes some grammarians to swoon. Personally, I can live with it.
I thought that 'kids eat free' was ok because it was short for 'free of charge'.
Personally I hate ' for free' in any context.
I think 'for nothing' or simply 'free' sound much better.
Is there any grammatical justification for this ingrained opinion?