Explain that they will not be permanently separated from you or the other parent. Reassure them that they can still see you, only in a different manner. To be a good parent, you need to be a healthy person. Make sure you are able to spend some quality time on yourself, as well as with your children. Counseling is also available on an individual or group basis.
Many schools also offer counseling for students.
This is a whole new adjustment period. It may take as many as five years before the new family unit is comfortable for all. So expect some bumpy roads. It's important to accept the stepchild or stepparent just as they are. Build a relationship around shared interests or skills and allow relationships to grow on their own.