This is complicated, in legal terms. She pled guilty to a lesser charge. A more serious charge, implying intent, is apparently possible.
But they are letting her return to Aust ... oh happy day for the Australian people.
Why was there a lesser charge, she murdered those two kids.
Under U.S. law you cannot charge someone with a different crime based on the same set of facts, or based on evidence that should reasonably have been available at the time to person was originally tried, once jeopardy has attached. A conviction on a lesser charge would mean jeopardy had attached.