How to handle disagreement with in-law about how to raise the kid?

As someone with in-laws from a different culture (my husband is Chinese), I feel very qualified to answer this. When we lived in China, my in-laws were helpful in many ways in regards to watching our children while we were at work. Here are some tips for handling clashes with the in-laws over parenting techniques.
 
Be respectful but firm
These are your kids, not theirs. When it comes to major rules and family values, what you and your spouse decide is best for the children is what should be enforced.
 
Don’t compromise on safety
If your in-laws do something that’s unsafe, don’t allow them to watch the children again until they can comply with your standards on safety. So if they habitually leave knives in places the kids can get them as my in-laws did, lay down the law.
 
Be flexible at times
So you don’t like that your kids are still up past bedtime when you return home. But if they’re otherwise well-cared for and having fun with their grandparents, let it slide particularly if it doesn’t happen often.
 
Share your ideals with them
They might not agree with your point of view, but you’re the parent and this is your family. Let them know what your values are and what you stand for and if they’re reasonable, they should at least respect that.
 
Apologize after arguments
And finally, don’t forget to make amends after an argument, particularly if you started it. It’s easy to get heated when your in-laws do something you don’t agree with. Keep the peace for the sake of your children. The in-laws will not live forever and one day when they are gone, you’ll regret not being more peaceful.