Is a notarized letter of permission acceptable to move my children out of state?

Technically you don't need anything right now to move out of state. Since you and your Soon-To-Be-Ex are actually in agreement with your taking the childen, it should not be a problem. You will only need to get permission from the court, once you file for divorce and the Automatic Temporary Restraining Orders (the ATROS) go into effect. The ATROS prevent people from moving the children, spending assets, and taking property where ownership is not yet decided. Those only apply when a Petition for Dissolution has been filed, and then served on the other party. Until a petition is filed, you and your STBX can agree to anything and you don't need anyone else's permission. It is a good idea to have things in writing, as once a case is filed, people tend to get testy, and memories can change as to what was agreed to.

Although you technically don't need even a letter of permission until one of you files for dissolution and the Automatic Temporary Restraining Orders kick in, you should have that notarized letter in hand when you leave the state. Things happen. Also: WHERE do you plan to file for divorce? If not in California, you could have a jurisdiction battle, particularly since children are involved. (Google "UCCJEA" -- Uniform Child Custody Jurisdiction Enforcement Act -- for details. Have aspirin handy.) You would be wise to consult a family attorney experienced in inter-state divorce and custody issues before you take the next step. (Disclaimer: This information about California law and procedures is based on the limited facts provided by the person asking the question and are neither privileged nor confidential and does not form an attorney-client relationship. Readers in other jurisdictions should be aware that California laws may differ from those of their state.) - edit