Little Maggie before she was kidnapped appeared to be living in a loving supportive home unless Susan was totally deceived. After the horrors she experienced, she was in another supportive loving home. So yes, she was severely traumatized, but she has also had a lot of loving support. She isn't totally unscathed, that is why she needs her therapy dog, Mouse. It is amazing what a therapy dog can do for someone suffering from severe PTSD, as any number of veterans who owe their lives and sanity to these wonderful animals can tell you. Most likely she has also blocked out the trauma she experienced, it all fit into a small time frame of a few days, she may have even been drugged or under a spell so she wouldn't fight what was about to happen to her. We also do not know if the Carpenters sought therapy for her.. That isn't to say it won't rear it's ugly head later on, she might even end up with multiple personalities.
Yeah, we're told she needs Mouse, but never shown it, in my opinion.
And fine, say she's a very resilient child--which I could buy! She should then act like, well, a child, which is to say she should pester people at inappropriate times, complain sometimes, be loud and annoying sometimes, etc. I am not a fiction writer (I wish I were), but she just doesn't feel right, and she's more of a plot point than a character.