Couldn't this also be imperative? Couldn't it also be translated "Girls, eat the jam!"? I'm mean really- otherwise it's a sentence that would never be used in real life.
With "voi" it wouldn't be an imperative. Imperative is mandatory without subject.
It could make sense in some contexts, like "Ragazze, voi mangiate la marmellata. Lasciate la cioccolata ai vostri fratelli" or something like that.