In principle radioactivity isn't out of question - there are tritium keychain lights, for example - but I'm unfamiliar with them. I imagine you could enclose a tritium vial in a piece of tough (so you won't accidentally bite into or break it) translucent plastic to get a light-diffusing surface, and mould the actual sugary part around it. The stick could be either embedded in the plastic or possibly made out of it? This sounds neat because there'd be no battery compartment that'd otherwise cast a shadow, unless it could be fitted into the stick itself. The main downside I see is that it wouldn't be nearly as bright as a LED.