Your role as a stepparent will be a huge factor in the success of your marriage. In fact, it's probably going to be the determining factor. Create a relationship with each child as an individual. Don't expect each child to treat you the same. For example, the oldest child may be the last to accept you.
Younger children adapt more easily.
Do this, even if the child support payment is late. Act happy about a child seeing the visiting co-parent. This is one of the most positive things you can do. Don't confuse yourself with a psychologist. When problems get sticky, call a parenting group or a counselor.