R.K.
He can't take your kids from you. This all a part of the domestic violence cycle. What he did was a federal offense, tampering with the U. S. Mail is a Federal offense. You can call the post office and they can send the Federal Authorities such as a Mail Inspector or the FBI. I'm not kidding here.
Look for a domestic violence shelter near you and get your ducks in a row and leave him. Talk to friends and seek help. You may have to store things at friends houses until you can find an apartment. Depending upon circumstances Section 8 can take years, in some areas the waiting list is 3-4 yrs long.
The good thing is at this time of year many places are hiring for the holidays and many of these jobs roll over to permenant.
Remember half of everything you own as a couple is YOURS no matter what he says. Half of all money in a joint account, half of the value of your home ect are yours. Don't let him bully you out of what is yours.