Ok, let me try this once more.
1. If she got something done in court, like getting him supervised visitation, without him knowing, she either lied or she didn't know where he was at the time. If she lied, you should be able to get a lawyer and refute that. (People who know she knew his whereabouts, any correspondence between them, phone records, etc.)
2. In the meantime, he should take every one of those supervised visits he can. Makes the court believe he's interested.
3.Document each and everytime she leaves the children with him. I don't believe she's breaking the law by doing this but if she's got him on supervised visitation, and then turns around and leaves the kids with him, that just shows she doesn't really believe he needs supervision. (In other words, it shows shes being vindictive)Make sure people see him with the children so you can have witnesses.
3. Document everything. If she sends them in clothes too small, take pictures of the kids in the clothes. Clothes inappropriate for the weather, make sure you have a camera that will record the date. If you can find out when the kids are left unsupervised, call the cops, call DFC, get it documented legally.
4. If the kids are in trouble at school, academically or behaviorally, get statements from teachers, principal, etc. Get copies of grades, report cards, write ups, everything.
5. MAKE SURE HE'S ABSOLUTELY CAUGHT UP ON THE CHILD SUPPORT!!!!
6. Find any and all witnesses that will testify that she is not putting the kids first. Make sure they are credible. No court is going to take the word of someone unreliable.
7. Check your state laws and record all phone conversations. It really would be better if these conversations were between her and your fiance, not her and you. Maybe if he understood that he could try again with her, after all, it is his kids at stake? Make SURE you do not provoke her in any way. Be nice even if it hurts. Let her be the one to look like an ass on tape. You may have to tell her your recording her. She may modify her behavior because of this, but even if she says something on tape and then doesn't do it, you've got her anyway. Lying is not a good charater assessment.
8. Have some sort of plan set up to show a judge in court. Be able to prove you can care for the children, that you can provide them with a home, stability, good childcare, etc... Show them your ready.
9. Get a lawyer. This is a MUST!
10. And finally, I say to you what everyone else has been trying to tell you. You gotta let go of the rage. You can be mad all you want to about the situation, but if you go into court and rant like you have on this board, no matter how true it may all be, the judge is going to listen for about two minutes and then decide you are a biased, vindictive girlfriend and quit listening. That may not be true, but that is what it will sound like and since the judge doesn't know you, he will go on what he hears from you. ALSO, you were right when you said that it would matter if you were a horrible person, because if you show this attitude they will believe that you will not foster the mother-child relationship, which is a MUST for a custodial parent. Custody has been changed for one parent bad mouthing and promoting bad feelings about the other. Its called Parental Alienation, and you don't want that tag.
So calm down, and get your ducks in a row. If you do what you need to, and be calm, if everything you said is true, you have a good chance of winning.