When my second was just about 2 months old, I had to go to Texas and help my sister plan her wedding and get married. I left my kids with Dad. It was SO hard for me. I knew I couldn't take them with me - oldest was in school and I knew I wouldn't have time to care for a 2 month old child while doing last minute wedding planning.
I made it seem as positive as possible for my oldest (at 2 months, oddly #2 didn't really care!) I promised to call every night at bedtime and promised I'd bring something home for them. It was nothing super swell - it was a t-shirt that said "Dallas" with a picture of a Precious Moments cowgirl on it.
In March last year, my husband and I came to Montana for a job interview. My Mom stayed with the kids - it was 2 nights. Again, promised to call every night and promised a gift. Bought them each a small stuffed animal that said "Montana" on it.
Your kids will miss you and you'll miss them. But, calling around bedtime helped and they were rewarded for their good behavior with their surprise.
Just make it sound as positive as possible. Tell them that they GET to stay with Daddy. Promise to call at least once a day and bring them a little something. Then, take a deep breath and go - it's hard, but the entire world won't stop, trust me. Go have fun and enjoy your kids that much more after your trip.