Make the best of it
Buddy up
Institute a buddy system with your spouse. Know the subtle signs that show they're nearing the end of their patience with your parents or theirs, and bail each other out as necessary. Make up excuses to run to the store for diapers, go for a walk or whatever feasible thing you can think of to get one or both of you out of the situation to cool off.
Make plans outside the house
Plan a fun family outing for the kids with your in-laws in tow. It's hard for anyone to misbehave when you're focused on having a good time with the kids. Even better, let your in-laws take the lead with the kids and take some candid photos of them that you can print, frame and send them home with as a gift.
Go to bed early
By the end of the day, even people you love can seem annoying. Rather than stay up because your father-in-law likes to watch "Law & Order" reruns until way after the kids are in bed, say goodnight, excuse yourself and turn in early. Make a habit of it starting with the first night and you can do it for the entire visit without seeming rude. Blame the kids for tiring you out, if you need an excuse. Going to bed early might also give your partner some alone time with their parents—which they'll appreciate—and you get quiet time on top of it. Win-win.
Set up a surprise activity
If your in-laws are visiting from out of town (or you're visiting them), surprise them with tickets to a museum or reservations at a restaurant they like, so that you can get a few hours apart. It'll be money well spent and your in-laws will thank you for your thoughtful gesture.
Focus on the kids
Kids pick up on stress, so do your best to give them all the snuggles and fun you can manage. As long as your in-laws (or your own parents) are good grandparents, don't cut them off during the holidays. Despite the fact that you might not get along with them, your kids didn't choose for their parents and grandparents not to get along.
Bonding time
If you approach your in-laws' holiday visit as a way for them to bond with your kids, it might take some of the pressure off of you and your partner. Let Grandma and Grandpa participate in bedtime routines, cook breakfast with the kids or whatever else they're comfortable taking on so they can spend quality time together. You can supervise if you really feel it's necessary, but don't hover. Give them some space.
