Given the absence of a single definitive source, here are the most plausible interpretations:
Sometimes, non-native English speakers use “hot” to mean “warm” literally. So “Sonnenfreunde, Kinder der Sonne – hot” might simply mean: “Sun lovers, children of the sun – (it’s) warm” — a comment on weather or a literal heatwave. However, the punctuation and word order suggest intentional stylishness.