Assess. Don't accuse.
Take a look at the situation instead of flying off the handle. When you witness the grandparents not following your parenting style, ask yourself a few questions to begin with:
Do they understand that they’re breaking your rules?
Will this impact your children’s expectations for the future?
Is this harming your children?
If it’s a one-off situation, or if it’s a small rule that won’t do much damage, you might want to let it slide. However, if grandparents are putting a child's safety at risk, deviating from an important diet, or throwing the child off-schedule, it may be time to schedule a hard conversation with the grandparents.
Realize your beliefs and values may be different from your parents.
One of the best things you can do when your parents deviate from your parenting style is to realize that it’s not usually intentional. Your parents grew up in a different era, with different values and practices and they’re prone to play those out when they’re with your kids. You may have very different values and beliefs than your parents, which may dictate how they interact with children. Try to remember that it's not personal, and you’ll be better off.
Give a reason for the rules.
When you’ve determined that it’s time to intervene, reinforce your rules with reasons. If grandparents don’t understand a reason for the rule, they may be less inclined to follow it. For example, if your child is on a gluten-free diet for health reasons and the grandparents keep deviating from that, it may help to explain that your child has a gluten intolerance and that eating foods with gluten causes digestive issues for her. Or explain that without a nap in the afternoon, your child becomes overly tired and cranky in the evening, causing more tension in the bedtime routine.
Reinforcing your rules with reasons allows your parents to feel less offended when an accusation is made, and it also helps the grandparents understand the importance of following your standards.