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.