I started as an MHT on a children's unit. Things I learned...
Never make a promise you can't keep.
Don't say you're going outside for free time if there is a possibility you won't.
Don't underestimate the strength of an angry child. If they are agitated do not go hands on on your own. A 6 year old can and will throw a chair.
You will have kids you like and you will have kids you can't stand to look at. Treat them the same. Odds are the kid you can't stand is that way for a reason.
When you set a limit, follow through.
Most of the kids are returning to the same homes that created their difficulties to begin with. You can not change that.
The thing that got me through those toughest days and was my guiding principle when I just wanted to scream at a kid...
The biggest impact you will have in those 3 to 21 days is not the med changes, behavioral interventions or amazing groups. You are teaching them that grownups don't have to be like they are at home. That an adult can be upset and not hurt you. That doing a bad thing doesn't make you a bad person, behaviors are a choice.
I now work with adult survivors of childhood trauma, what I can tell you from that perspective is those who use drugs and alcohol report beginning between the ages of 7 and 12 so don't skip that part of your nursing assessment even though it may seem ridiculous. Many had traumatic pregnancies in late childhood/early adolescence. Remember a girl can get pregnant prior to starting her menses. That perfect families can have horrific secrets, trust your gut. If an 8 year old is self harming she is being hurt.
As far as books to read I would find out the philosophy/theoretical framework of your unit, ask your unit manager, psychologists or Attending if they have any recommendations. Also ask about your behavior management program. My hospital used 1-2-3 Magic which was great so knowing the basics of your management technique would be helpful.
Remember to have fun! It's been 10 years since I've worked the inpt children's unit and I still miss those crazy kids. I love my job now but always say that the only thing I would leave it for is to return to a children's unit.