Dh and i went on our honeymoon to Italy and left DD with my mum for a week and my MIL for the other. I left a list of bedtime routines, emergency contacts and meal ideas.
DD had a great time and we got daily updates via email from MIL and SMS from my mum. It was great.
I was really worried about leaving her for so long but it all worked out fine. Though do expect yourself to cry when you leave (DH did too) and i cried alot over the emails (though i was preg with DS at that stage).
You must trust your inlaws to look after your kids or you wouldnt be doing it. Just try to relax about it...the kids will get spoiled rotten!!