you don't need antifreeze for an air-cooled rad unless it's sitting outside through winter. distilled water should be fine; however, you'll probably still need/want something to prevent corrosion and possibly a surfactant (water wetter type stuff). not sure how toxic these sorts of things are, but at least they probably won't be delicious like antifreeze.
surfactants are basically soap, i don't think most will cause anything worse than diarrhea.
i don't own a reserator myself and i don't know what it's made out of, but even with a 100% copper system, you'd still want an anticorrosive, perhaps even more so - pure DI water will literally dissolve copper after a relatively short while. it seems pretty sturdy though, and having the res, rad and pump all in one eliminates at least 4 points of failure; if you're careful with all your fittings i'd guess your pets would be more likely to get hurt by having it fall on them.