I was seriously abused as a kid. I was beaten almost daily. I was probably raped by family friends as a very young child. My parents each had their own addictions - alcohol, gambling, shopping, etc. For a long time we lived paycheck-to-paycheck, and literally there would be several day stretches where I just couldn't eat because we had no money for food. The shopping trip the day before payday was like Christmas. I could eat!
I never thought anything was wrong until I was in my late teens. To the outside world I was a normal kid doing normal kid things. CPS never got involved, nobody said anything. When I later talked to my former best friend who I hadn't seen since middle school, I told her what happened in my childhood. She said she thought something was amiss, but never could tell what.
Anyway, the point is that horrific things can happen to a child, but children don't know any better. Child abuse may be that kid's reality and they will think that's how it is for all kids. It's just their normal. Many cases of child abuse aren't discovered until the kid says something odd to a teacher or relative, but to the child they are naive and don't know it's wrong. Child abusers go out of their way to stop their children from talking to others. Look at the one case recently in California where the parents had the kids tied up for decades and the kids were obedient, most not even wanting to leave the abuse.
I think that's why Complex-PTSD and DID don't show up until the early-to-mid 20's for most people. Kids just don't know. They don't know that what happened was wrong or abnormal. It's not until they start comparing their childhood to others when they feel robbed, and then the mental illness sets in.
EDIT: Point is, a child who may be MKU abused will probably think nothing is abnormal and they may act odd, but nothing to throw up serious red flags.
Very sad.