Talk to her and see if there is a way both of you can have your plan. Work together.
Find a baby sitter and go out with your friend.
See if your friend can go out another night.
However, you can't just go with thd friend and screw her plans over when she let you know of them in advance, that wouldn't be respectful. Nothing wrong with wanting to see your friend but you have to work with her still because of the children. If her taking all Saturdays will conflict with you work it out with her to maybe swap each week.
Accept what is, let go of what was, and have faith in what will be
