Relax. Breathe. You are fine. Your kids are fine. HE is exactly were he needs to be.
When I feel like panicking, I force myself to stay in the now. I don't allow myself to over think things or worry about what could happen, what might happen or what I should do.
These things have a way of working themselves out. Repeat the serenity prayer to yourself. Try to live by it.
And you don't have to tell your kids anything until you are ready. You can tell them "daddy is where he needs to be right now to get help with his problems but don't worry. I'm here. I'm not going anywhere. We will be just fine."
Patience really can be a virtue in these situations.