One of the most biggest changes that divorced couples face in their co-parenting relationship is when one of them gets a new significant other who becomes part of the kids' lives. Despite whatever conflicted feelings this may bring up for you, it's important to encourage your kids to accept and get along with this new person. After all, you'd want your ex to do the same for you. The more adults your kids have in their lives who care about them, the better off they'll be.
If you're fortunate, your relationship with your ex is good enough that he or she tells you about this person. However, you may hear about the new significant other from your kids. Either way, you have every right to meet anyone who's going to be part of your kids' lives. If your ex doesn't want to arrange it, you should find a way to make it happen.
No matter how you and the new boyfriend or girlfriend feel about each other, it's best for the kids if you can have a cordial relationship. This can sometimes be easier if the person is new in your ex's life. It can be more difficult if he or she is someone who was already around -- either in a relationship with your ex while you were still married or perhaps a mutual friend or acquaintance of yours as a couple. Either way, keep any negative thoughts, feelings and words away from your kids.
As long as the new significant other doesn't infringe on your role as a parent, do anything that's harmful to the kids, consume your ex's attention to the point where the kids are being neglected or make a habit of taking over your ex's responsibilities in custody and visitation exchanges, it will be healthier for everyone if you can find a way to accept this new person in your family's life.
If you observe or hear of concerning behavior or issues that you're unable to resolve with your co-parent, you may want to talk with your family law attorney. Your attorney can work with you to seek to modify your parenting plan or enforce provisions that are already in it.
Source: MommaSaid.net, "PARENTING TOGETHER APART: When Your Ex Has a New Significant Other," Brette Sember, accessed April 26, 2018