We just took a trip to Japan and China for two weeks, leaving our three kids with grandma. That was the longest, and furthest we'd even been away, so we were worried. Some great things that worked for us were:
1-we made a countdown chain for the kids. We made it with them before they left, and then they could cut one off each day we were gone... it just helped them to see there wasn't that much time left!
2-We also left something for them each day. Some days it was a note (but two weeks of notes would get old!) so we also left new activties to do (fun cupcake molds, new marker and coloring books..ect) giving them something to take their mind off us being gone, and also giving grandma things to do with them. It really was a hit. One day we even just gave them all fun breakfast bowls. I think this was their favorite of them all!
3-The BEST was Skype. You can sign up for free and all you need is two web cams. You can buy really cheap ones if you don't already have some. You just hook those up to the computer, download skype (this is of course assuming you'll have a computer with you) on both computers and you can talk within the states for FREE! From Japan in was only $.03/minute. You can't beat that really. So we just made sure to talk to them every day. It really helped the yougest one to be able to see us! We were with a big group, and every single person over there with kids was doing the same thing. I don't know how we would have done it without... really!
4. The last tip is to maybe line up some play dates for the kids to give everyone a break.
Good luck, and don't sweat it. I'm sure things will go great!!! Besides, it give the kids and you mom some good bonding time!