I don't know why they seem to think others where involved either. It doesn't take much intelligence to keep a 9 year old child drugged, in fear and tied to a bed. The hard bit would be getting her to the house without anyone seeing her, and I think that was pure luck. It wasn't some mastermind plan, they got caught in the end and were suspected long before that.
I couldn't believe Julie expects Shannon to come back to the estate, that was a bit jaw dropping. I don't believe for a second she is thinking of 'the kids' (18 year old young adults more like) she wants her back for her own curiosity, she is too overinvested in this still and she would love to get her hands on Shannon to fill in some blanks and have some gossip fodder. She would be straight onto the newspapers trying to extend that 15 mins if she heard any tidbit of info on Shannon, her ordeal or her new identity.