Grandparents love nothing more than to play with their grandkids. And so, this goes without saying that you should let your children spend quality time with your in-laws if you happen to live separately. It can be twice or thrice in a week or even for a few hours every day. This will strengthen your ties eventually with your in-laws.
Remember, these rules are only for situations you feel you are at emotional loggerheads with your in-laws. Otherwise, the best bet always is to give all the love and care that you can and treat them like your own parents.