I am assuming your little one is bottle fed ? As long as the kids have good relationships with their aunt and grandparent, you should be fine, although juggling what is basically a newborn baby and the other two may be a bit challenging. . . .
Why not invite them to come over and see how your household runs ahead of time ? You can leave a list of what the kids like to eat, etc., and you can leave a written schedule of a "typical" day, but don't expect it to be followed . . . when someone else babysits your children, you have to expect that they will bring their own style to the position, and they will parent your children as they see fit. If you don't trust them to do that, or if you are personally tied to a very hard and fast schedule, then I recommend you stay home.
Children are very adaptable, and they learn that there are different rules when they are in different households. I have also learned that no matter which Grandparent (and I've dealt with 4 teams of them) I deal with, the grandparent is going to do what they want to do, what they think is best, and you can like it or lump it. It can be frustrating, but when you decide to leave your children with another adult, you cannot expect them to be you. They will be themselves, and they will build their own relationship with the kids. Hopefully they will try to stick to some semblance of your routine, for their own sanity, but if they don't, you will simply have to re-organize the family when you get home.
In terms of preparing . . . Why not write a picture book ? My sister-in-law did that for her kids when they were all going on a plane for the first time to visit great grandparents in FL whom they'd never met.
You can write the story, that Mommy and Daddy are going on a trip. They will be going to Aruba, and I'm sure you can get pix of that off the web. You can show you guys getting on the plane and flying away . . .
You can then write about their aunt staying with them, and their Grandparents doing the same, and how much fun they will have together. And then, after 6 days, who the plane again, and have the story end with big hugs when Mommy and Daddy came home, and how excited they were to have their children back again.
Give them simple information, but give them enough information, so your relatives can read the story to them while you are away, and you can read the story enough in advance, and regularly, so it will sink in. That way, even when they miss you, they will know, "Mommy and Daddy are coming back."
If there are things your children like to do -- is there a children's museum near you or a zoo ?? Those are things that they can do to make the time without you special for them. ??
Have a fun trip !