All you do, is personally thank her and tell her.
Directly.
She knew how your son is and still took him. To give you a break.
She wanted to. You did not have to twist her arm and force her.
She is nice and did it out of thoughtfulness.
So, just thank her.
With heartfelt words. Or a thing of cookies.
She is your friend. She knows you are appreciative because she knows you.
However, some people do nice things for others, and then they expect the other person to be SOOOOOOO grateful and demonstrative of it and they want to hang it over the other person's head and want to keep hearing "thank you so much for..." all the damn time.
So with people like that, they are not being helpful out of thoughtfulness, but to assuage their own, sense of ego over others.
I only say this because I know someone like that.
Anyway, in your case, all you have to do is say thank you to her. She is your friend. You don't have to do any favors for her to thank her. Just saying a heartfelt thank you, is enough.
A friend, understands.