C.G.
I travel for work and have learned a couple valuable lessons.
First, be honest with them about what's going to happen. This is where you're going, who you'll be staying wtih, etc. I took my son outside one afternoon and showed him an airplane up in the sky - and told him that mommy would be getting on one and going away for a while. Especially for the youngest, keep the element of surprise out of it - and build up any differences. IE: My sister in law has to take my 4 year old to daycare in teh mornings when I'm out of town - so we get him excited about that.
Second - emphasise the positives - IE: You are going to have so much fun when it's just you & daddy! Encourage daddy (or whoever will be watching the kids) to do something fun with the kids so they can have a great story to share with you too!
Third - DO NOT call at bedtime. Bedtime = Tired Kids = EMOTIONS. My son would be just fine if I called right after daycare - maybe just before dinner time. If I forgot and didnt call until bed time - it was not pretty.
Lastly - Get regular updates from whoever has them - if they did something great while you were gone (helped daddy clean up the house, helped Grandma fold laundry, etc) bring a small "treat" from where you are. A little stuffed palm tree from the airport or something goofy - so that you can show them how proud you are of what a great job they did (and cite the example).
I've travelled since my son was about 18 months old - and he's just fine - as yours will be. However, if it is the first time away, be prepared for a few "I want to talk to mommy!' phone calls. Good luck, and ENJOY YOUR VACATION!!!