This is an interesting premise! I don't think I've seen anything like this before. It's hard to really critique your grammar since you're supposed to be writing from the POV of a ten year old who definitely wouldn't be perfect. Therefore, I'll just point some stuff out, and you can decide whether or not to change it.
-In the third sentence, you don't need a comma before "there will be..."
-You need a comma after school in "...like my teacher in school, and he..."
-In the next line, you also need a comma after "door", after "first", after "home", and after" weren't"
-You also need a comma after "mommy" in the last line.
-I'm just confused on one point: you say "I hope you find this soon", so does that mean that she is leaving this diary behind? Or sending the letter somehow?
-In chapter two, you need a comma after "...very sad and scared"
-In the beginning of chapter three, you need a comma after "...some food"
-Later on in chapter three, you need a comma after "...in the building"
-Comma after "...really nervous"
-Comma after "...started shaking"
-I'm going to stop pointing out the run ons, but the general rule has to do with "FANBOYS" conjunctions. (e.g. for, and, nor, but, or, yet, so). If you have one of these and what follows is a complete sentence in itself, you need a comma before the conjunction. So "We got down and I started crying." needs to be "We got down, and I started crying."
-In chapter four, you have "We were lead..." it should be "led"
-Chapter five, it should be, "Sorry I haven't written in a while."
-Chapter six, "...if she rat him out." should be "ratted"
-"Man #1: A inconsiderate..." should be "an inconsiderate..."
-I don't understand the line, "they know I can kill". Have there been any instances where she did kill?
Wow. You've taken on a very difficult topic and I think you've pulled it off very well. It's moving, and I like how you've changed the author's voice to reflect her growing up. The only thing is that I don't find it entirely plausible that they would send anyone home at all. There's far too much of a chance that they'd go to the police. Other than that, though, you're doing great, definitely keep it going (just watch run ons ;) )!!