Why exactly do Teeny and Sam hang out with Chrissy when they don't seem to care for her nearly as much as Roberta? They make fun of her the most (to her face and behind her back), talk to her less than each other and Roberta and when they do it's usually arguing or scoffing at her naivete. Just because she's letting them use her yard for the treehouse? Because Roberta likes her and they like Roberta? They did come as adults when she asked, but as kids, I didn't see much love between those guys, but might be YMMV.
How are the other girls less naive (though not well-knowledgeable) about sexual matters? Well Roberta has three older brothers who'd probably talk about sex in front of her, Teeny left on her own seeks information from issues of Cosmopolitan and the movies, and Sam reads a lot and likely her parents aren't as prudish as Chrissy.
