You answered it yourself. Parents don't know/care, their kids bare the brunt.

Taxing isn't ideal and is a blunt instrument at best - but I'm not convinced there's a better (working) alternative.

I don't think it is a blunt instrument, in fact I'd say the complete opposite. It's been a nudge of industry in a direction that's better for society, in a way that keeps that playing field level for different manufacturers, reduced their overall tax burden and not cost consumers anything. All this while avoiding any harsh ban, you can still buy high sugar drinks if you wish.