I would start off by saying that you did agree to the hours and it's only been a month ~ having said that you can't give your own family time 100% and it's draining you. Honesty is the best policy.
Give them the option of taking care of them for the summer and or until they find another provider if you want to end it sooner than later.

Just keep in mind that this family did what they had to do. As long as you do what you feel is fair and able to maintain your own sanity- you have to do what is best for you.
Good luck!
I've done the long hours when I first opened plus our own kids and I was EXHAUSTED! I totally get why your looking to end the hours. It's too much when the job is very demanding. Don't burn yourself out, easy to do in this business.