I guess you can look at it two ways.
1.She lied to you then, or 2.she has finally come to the point in her life and marriage where she wants to be fully honest with you.
She may have withheld details when dating you thinking that revealing them would scare you off. After having been married to you, she probably has had those details eating on her for years. Perhaps she finally reached the point that she wanted to clear her conscience and be open with you. If she has been completely faithful to you since marriage, then you can potentially use this as an experience to start a new level of trust.
With kids involved, it is a more complex situation. If she is willing to commit herself to you from this point on, then it would be in your children's, and your best interest to forgive her and move on. Counseling may help you with this.
She could have easily kept all of this a secret, and life would have gone on with you being ignorant. Would you have preferred that? Or are you glad that the truth is finally out, and she is trying to be honest.
Yes, it would have been good to be honest from the start, but if you have enjoyed your time being married to her, and love her and your children, then focus on your future together and not her past.