Here is a question I'm often asked: What point, if at all, should stepparents start disciplining their spouse's children? My answer? As soon as you move in together or are married. Why? Because as soon as the stepparent is alone with the kids in the course of normal family life, the kids will test her or his authority. Kids, being kids, will see what they can get away with -- not because they are defiant, but because they need to know where the limits really are. If the stepparent doesn't have the total support of the bio-parent to be a disciplinarian in the kids' lives, it's a set up. "I don't have to do what you say. You're not my real father/mother!" is a line that every stepchild worth their salt tries. And then the conflict begins. Unresolved, it often ends with the break-up of the couple.
How can a couple avoid this mess? By making sure they talk things through before they marry or move in together. It's critical. People with children are parents. Committing to someone with children means that you are a parent, too. Inviting someone into your children's daily lives puts that person in a parental role, regardless of what that person is called.
For the purposes of this article, I'm going to assume that all the adults involved with the kids are well-intentioned -- essentially, healthy and helpful parent figures. If someone is abusive or inappropriate, there are, of course, different considerations.
How to become effective co-parents:
1) Don't introduce romantic partners to your kids until you are sure that you are going to be a committed couple. If you are the romantic partner of someone with kids, be helpful in this. Don't get involved with the children until you know you are serious about the parent. If a mom or dad keeps bringing in new loves and then breaking up with them, the children often conclude that love doesn't stay. This can discourage them from finding stable relationships when they grow up.
2) Once you have decided to marry or move-in together, the two of you need to have a long talk (actually many long talks) about your philosophy on discipline. Discipline, by the way, does not mean reward and punishment. It means the method by which you will teach the children and teens to live by family rules and negotiate when they don't like them. It's absolutely critical that you be on the same page about this. What are your "house rules"? How are you as a couple going to encourage and discourage behavior? What consequences will be imposed if a child breaks a rule? How will positive behavior be supported?
3) Present the rules to the kids together as a couple. The bio-parent needs to be clear that the step-parent or partner is acting as the bio-parent's eyes and ears and has the bio-parent's full support.
4) Expect the kids to test you. Deal with it with humor. Once the kids understand that you both mean it when the non-bio-parent enforces the rules, they will settle down.
5) Whenever there is conflict among the adults (bio-parents and steps), get together to work it out. Don't act out in front of the children or ask the kids to carry messages for you. When adults deal with each other with respect, even when they don't agree, it models helpful and mature conflict resolution for the kids. Further, it prevents a situation where the kids feel they have to choose between people they love and need.
6) Work with your partner to develop a positive and caring relationship with the children. Life can't be all about discipline. There needs to be a balance between the good times and the day-to-day life of overseeing homework, cooking and cleaning up together, reading stories, family outings etc.