A 'tea party' (in this context - in the context of American history it's something quite different) is a game played by children where they pretend to have a meal, or quite often a picnic, with their toys (dolls, toy animals, etc). A 'den' (in this context) is a place where children meet to play games -often a with the idea that it's a secret, or a base for a gang (in a childlike sense). So the whole sentence wants to convey the idea that the experience is bit like a child's adventure or play.

This doesn't read like it was written by a native speaker. Where did you find it? "Her office was in a caravan parked." isn't grammatically incorrect, but it doesn't make sense. Could it be mistranslated or miscopied? That said, "tea party" is a game that little kids (usually girls) play. They sit around in a circle with their friends and/or dolls and stuffed animals. Then they pretend to drink tea and talk.