Hi Misty,
First of all, I do not think you're paranoid. You have every reason to be concerned about your kids being so far away for so long with a dad who may binge drink while they're there.
Your kids may already understand more than you think. It might be helpful to sit them down and explain to them that he has a problem, let them know that you're just not sure how well he's doing these things, and to be sure to call you or your girlfriend if anything happens.
I'm sure other moms will have better input. Your concern is valid and I hope things turn out ok.
Hugs,
JG