OP, I want you to think.. REALLY think... in the long term. Don't think about his other child. Think about you and your children. The reason I'm saying that, is because staying with him after the affair can put you into a precarious situation I'm not sure you've considered.
If the other Mom files for child support, he's going to be paying it. If you later divorce, your children would get a markedly smaller amount of child support. When it comes to child support, whoever files first usually ends up with the better "deal".
You're already in a lot of pain, but please consider: your kids will pick up on the tension in the household. Might it not be a better idea to get out now and help yourselves heal?