Need some more advice.... I think My niceness is being taken advantage of, the Same family that I have had on going issues with in the past seems to think I don't need to be paid on time.

This family has 2 young children in my care and because they pay $1400/month I agreed on the 30th and 15th payment, payment has been late at times but they always given me days notice and always pays my late fee. I don't allow any of my other family's to pay late ever and make sure payment is in on the 1st!!
Well Yesterday being the 15 I should have received an email transfer with payment, nothing came and around 5pm last night I send mom a text asking if she sent payment yet.
This was her response "Hey. I know I haven't sent it yet. Unfortunately I can't send it till Wednesday. I'm really sorry, I know that it's $10 for each day that it is late and I will include the late charges on Wednesday"
No notice or anything was given to me... I'm so tired of dealing with this family, I know I should terminate them tomorrow but that's a big loss of income for me! Any advice on what I should say to mom on Tuesday?
In feeling so disrespected and taken advantage of!!