What do you do if you are married but fall in love with someone else who is not married but both have children?

I was also in this situation and it doesn't always work out that way. Both people in the marriage have to be in love and committed to each other to make the marriage work. If that is not present, it is very difficult to make things work out and the children do not benefit either. Unhappy partners make unhappy children. It depends on the situation between the two people.