It’s typically best for children whose parents are separated or divorced when those parents have consistent rules and expectations of them across their households. These can be negotiated and detailed in your parenting plan.
The reality, however, is that one parent is typically more lenient than the other, and parents often are more conscientious about enforcing some rules than others. Likely, this is the way it was when you were together, so there’s no reason to expect that you can parent entirely consistently now that you’re apart.
Kids can adjust to varying rules and expectations
Children learn to deal with different sets of rules depending on where they are. They know they have to behave differently at school or during religious services than they do at home. They may know it’s okay to occasionally swear around their parents, but not when they’re at grandma’s house. They may not always say “please” and “thank you” to their parents, but they always do when they’re at a friend’s home.
If your co-parent has somewhat different rules, you may need to occasionally remind your child of your rules when they’re in your house. Be sure to reward them with praise – especially if they’re very young and this doesn’t come easily to them yet.
You may even want to post some rules on a handmade sign or whiteboard as a reminder. Make them “house rules” that you abide by also (like putting your stuff away, washing hands before meals and so forth).
Unless your co-parent is allowing your child to do something that’s dangerous or otherwise potentially harmful to them, there’s not much you can do besides talk to them about your concerns. Emphasize your child’s well-being rather than why you want a particular rule followed.
Be sure that you don’t criticize your co-parent’s rules (or lack of them) to your child. You can just explain (if they complain) that this is how things are done in this house. As noted, detailing some agreed-on rules and expectations for your child in your parenting plan can help. If you feel you need to add to or modify your current plan, it’s wise to have legal guidance.