If you can't file before she's departing, no there's nothing you can do. But you should contact a local family lawyer and file a petition with the family court alleging custodial interference.
you could file for emergency relief by order to show cause. Be prepared to answer specific questions about how you know she is planning on leaving. you will need to file for custody of the youngest. I am surprised that your prior custody order does not address relocation. The biggest hurdle you have will be that you are "anticipating" iinterference. So the more concrete and detailed you can be; the better.
Sign up to receive a 3-part series of useful information and advice about child custody law.