You didn't mention it.. so I'm not sure if it will work.
Would it be possible for the kids to stay with dad over night? You said they stay with him on weekends so I don't know if he is close enough for it to work or not. That might be easier for the kids. I hated it when I had to ask my ex he always made a big deal out if it, but it was easiest for the kids.
Just try to make it a "fun" night, movie and pizza and a slumber party in the living room with who they are staying with might make it easier. Call them before they go to bed and possibly in the morning to tell them good morning and let them know again that you will see them later.
Maybe get them a stuffed animal the day before you leave and tell them every time they miss you they can hug it and you will be right next to their heart.
I would try not to make it a big deal when leaving or they will have a harder time.
Good luck and it won't be so bad once you leave, worrying about it before hand and while there will just make it harder on you.