First, don't freak out. Trust me, it really doesn't help anything in the long run, and kids pick up on your stress.
What's your branch? Is this your first deployment? If so, there are some classes out there that'll help. Make sure your kiddos are registered with CYS (Child-Youth Services), which will give you a few free hours of babysitting a month, which can really come in handy if you just need an hour to yourself to do nothing other than breathe.
Also make sure you meet a few people in his unit. Get in touch with the FRG (Family Readiness Group), know who the Rear Detachment people are and who to call for what. It can all seem pretty daunting at first, but it's completely doable.
My kiddos are really attached to their daddy too, and the first few weeks can feel like absolute hell. But what we've done, is each kid has a special stuffed animal from daddy, and we spray it with his cologne. Pictures everywhere, and I had my husband write a couple letters before he left that I could read to the kids whenever they were feeling down.
Another great thing, is depending on where he's going, there's a very good chance he'll have some pretty decent internet access. That was a life saver. Webcam, voice chats, emails. It was an amazing help. The kids could see daddy and know he was okay.
Above all, take a deep breath, let it out slowly, and know you can do this. Everything will be fine. You'll just be a single parent with a mysterious benefactor (family joke), for awhile. But you CAN do this.
It gets easier to shift into the deployment routine each time, we're on our...sixth. Anyway, if you have any questions, or just want to talk, feel free to PM me!