I disagree and would suggest that you speak to a Rav before you say anything negative at all to your mother-in-law. Do not get into any conflict with her. You have as much of an obligation of kibud av v’em with in-laws as with parents. Set the rules with your own children and discuss a plan with your wife. But be as polite as possible with your in-laws. Make your visits shorter and shorter if that is what is necessary to control your children.
Personally I follow my daughter and daughers-in-law’s rules. If the kids ask for nosh or want to watch TV and their mommies are in my home and tell them they have to ask their mommies. If they are not in my home I ask what do their mommies allow. I know what their mommies allow, but I want to hear it from the kids. Each mother runs their home a little differently so I have to remember the rules for the different homes. I teach my grandchildren that they have to listen to their mommies no matter where they are. Sometimes I let them take snacks home to show mommy and ask them if they can have it for dessert after supper.
When my neighbors kids used to come in to watch tv, I told them your mom doesn’t want you to watch tv.