One thing I learned when babysitting high energy kids, is that generally, to get them to calm down, you have to be calm as well. If you've ever watched the Dog Whisperer, it's the same basic ideas on how to get them to behave and calm down and relax; is to be calm and assertive. I know it sounds weird comparing little kids to pets XD but it can be pretty similar sometimes.
But with little kids, sometimes its also best to just let them let out their energy (without destroying things of course) and then they'll calm down. So I like Gawqueenpenguin's idea of bringing somethings with you. Bring some board games; but if they get bored with that, jave a backup plan of some sort of fun hands-on crafty thing. And have a couple movies or something too, so once they start to settle down a little bit, you can pop it in and let them relax.
Anyways, good luck and have fun :)