I think Esme wanting them books says alot. She's obviously intrigued by sex ect but it probably isn't something she learns about at home and she has no friends to discuss it with, and I'm assuming Isabelle doesn't know a whole lot more about it, so this is her way of finding out more.
When I think back to that age, I can't imagine going through it and all the teenage feelings that come with it, without anyone to talk to. Probably around 90% of our conversations at that age were about boys/boyfriends and sex even though most of us were far off doing anything ourselves. It was all about who we had a crush on (which changed daily), what we wanted to do with them (without really knowing what half of it involved), who is going out with each other and what they are doing, who should go out with each other, passing on any knowledge we had read/heard/seen.
She is in a difficult situation with no friends, family that have revealed other secrets (or made up things) online so can't really be trusted, and the only outsiders she talks to are randoms on Roblox who are the absolute worst people to talk about that with because they could be anyone and it could lead to a dangerous situation.
It's yet another thing that only gets worse with the younger children as they grow up so isolated, Isabelle at least had some teen years in school and managed to have some contact with old friends after leaving, even if only occasionally. The others have no one at all and no chance of finding a friend. Despite this we can all see how immature Isabelle is but the others will only be worse.