First, let me say, I feel your pain! My heart breaks for you and I understand how devastating a change like this is. My husband is a Marine and I have experienced the pain of separation. We have mostly lived in the same town where all of my family is, but we have had two tours that took us about 400 miles from "home," and we will likely be moving across the country in a few years. Since the kids came along (they are now 3 and 2) the separation has been even harder. My boys are very close to my parents, as well as my brothers and sisters and all of their kids. If a move to Texas is not a possibility for you (I have spent a little bit of time in Texas, and I can tell you, as a California girl, I LOVE Texas. The people are great and it is a wonderful state. Texans are proud for good reason! The climate is different from California, but to be close to family, it would be worth it for me) - anyway, if moving there is not an option, these are some things that we have done to cope with separation from my family (and my husband's family, they are spread out all over the US so all we know is long distance contact with them).
We have cell phones with "free" long distance and unlimited nights/weekends, we also have the same carrier as many in my family so we can talk anytime to them for free. This might be a good way for your husband to stay close to your dad. We keep the family close by having pictures up, as well as little photo albums that my boys can look through whenever they like. My 3 year old is quite a talker now and loves to talk to his various grandparents on the phone, even the ones that live only 3 miles away currently. My 2 year old is just starting to show a little interest in the phone because he sees his big brother doing it. He is not as social and verbal as his big brother, but I don't think it will be too long before he can talk to the long distance family over the phone.
The grandparents who live far away try to visit once or twice a year. When we were living only 400 miles from my family, my parents would come visit every couple of months, or we would go visit them. Texas is a little far for a road trip, but if your parents are able to come out to visit you as often as they can, even if just for a weekend, and if you can handle the travel with two little ones, you could try to visit them once a year, or whatever you are able to manage. The time does go by fast, and even with such limited contact, my boys know all of their grandparents very well. They are closest to my parents, of course, because we have lived close to them, but even the other grandparents that they only see once or twice a year, they are close to them also. We look at pictures and talk about them, talk to them on the phone, so they are a part of our boys' lives that way.
My 2 year old was born while we were not living close to any family, so all he ever knew for the first 15 months of his life, was short visits to and from grandparents, looking at pictures and talking on the phone (which he didn't do much of then). Still, he recognizes all of his grandparents when he sees them, both in person and in pictures, and he gets excited. And since we have lived close to my parents for the last several months, he seems to have formed a very close, special bond with my mom.
This sort of separation is something my kids are used to, I suppose. Since your daughter is not used to this, I imagine it will be a tough adjustment for her. But kids are really resilient. If you can be calm and confident when you explain the circumstances to her, that may help a little. Distraction is also a great tool to use at this age, so if you can give her something to look forward to or be excited about, that can help. For now, if you don't know about any visiting plans, you can tell her that Grandma and Papa are moving far away, but we still get to talk to them on the phone. (I always say things like that as if it is really exciting, and my boys seem to catch my "mood").
I really don't think there are any easy answers for this, but if you are determined to make the best out of it, I think everything will work out fine. It will be a difficult adjustment, and it may not be life as you dreamed it, but you will adjust and it will become "life - as you know it." And hopefully you will have many visits to look forward to!