It’s not easy to be a teenager who knows that they were the «oops» baby, especially when one of their parents has «moved on» and started a whole new family. There’s a right and a wrong way to have kids in two different marriages, and it’s of the utmost importance that you make sure your kids from your first marriage don’t feel discarded. It shouldn’t feel like «This is my wife, my kids, AND my kids from my first marriage.» All your kids should be treated with dignity and respect, no matter how you feel about their other parent. 

All parents make mistakes, and there will always be situations where they, however unintentionally, treat one kid differently from another. A parent might think that they don’t have to give as much love and attention to a 15-year-old as they do to a 5-year-old, but that is the furthest from the truth.