Just wanted to let you know about our experience with this. My mother and aunt live in the same house, and together they've been my son's only caregivers besides dh and me. (They're great with him, and are completely willing to follow my attachment parenting style, even though it's not how they were raised.) When ds was 18 months old or so, they'd been watching him for over a year on a very regular basis (I work 19 hours a week, and they'd also watched him for date nights for us). They even watced him for an extended length of time when we saw the Lord of the Rings trilogy, which lasted all day and into the evening. By the time we got back to get him, he'd already gone to sleep for the night. Since he was so used to them and their home, seemed so comfortable and happy there, and was able to fall asleep for the night, they offered to give us our first overnight date for our anniversary. We spent the night at the coast.
What's important to note, however, is that it didn't work out well at all. He did fine while awake, he did fine going to sleep (cosleeping with my Aunt Leslie in her bed, where he was used to napping), but when he woke up, he was traumatized. When he was asleep, he'd forgotten all about being at their house. He'd never woken up in the middle of the night anywhere but with me, and this time I wasn't there! He cried and cried, for over an hour, more than once during the course of the night.
When we got back the next day, he was taking an early nap (probably because of all the sleep he'd lost that night, poor little guy). When he woke up to seeing me, he didn't react how I expected at all (I thought I'd see happiness and big smiles). Upon seeing me, he burst into tears, and he literally cried for an hour in my arms. He wouldn't even nurse at first. He finally did nurse, then, and he nursed for a long, long time. For weeks afterward, he alternated between being very clingy and pushing me away. I think he honestly must have thought I was gone forever. It certainly gave me a taste of how hard it would be for him and his caregivers if anything did happen to me.
Anyway, I completely know how you feel about wanting a vacation just the two of you--you can feel just desperate for some alone time. But I agree with the other posters that this time can wait until our little ones are really ready. DH just got a really good job (we've been struggling college students for years), and the first (well, almost the first) thought that entered my head was, hey, we can finally afford to spend a night at that hotel we honeymooned at! But then, of course, I realized that my dd is only 7 months old now, and way, way too young to be away from me, even on our anniversary 3 months from now. I figure, when I'm really confident that they're both ready (probably when my youngest is 2 or 3), we can try it, but we'll do a practice run first. We'll do an overnight at HOME, so if a little one gets upset during the night, we can just drive over and pick him/her up. I wouldn't want to have to drive all the way home from the coast in the middle of the night, but honestly, I think that's what we should have done last time (if we'd known what was going on). It would have spared him some trauma.
I know you said your little one sleeps through the night, but there's no way to know if that will be the case in an unfamiliar environment. Anyway, I hope this helps you with your decision, and that you and your kids have a wonderful weekend, whatever you decide.
Mother of three and strong advocate of being KIND to each other. (No one is going to learn how to be a better mother by your telling her she makes you want to throw up.)