I am wondering where it comes from. I don't think it's from the ass. As far as I could tell it was pretty translucent. If it was from the ass I would think it would be brown, or at least off yellow.
If she's old enough to be blushing so heavily at that scene she's probably hit puberty (not to mention she does have some boobs, though very small).
Thus she'd be able to cum. Therefore I'd say it'd be safe to assume her "frog juice" would either be cum or piss.