Ex husband battle......

My ex and I share 50/50 custody of our two daughters. For the most part we are very cordial and do what’s best for the kids. I found out the other day that he is leaving my 9yo at home alone for 4 hours 3 mornings a week on his weeks. He knows full well that I am NOT ok with this. She’s 9yo for goodness sake!

I guess my question is do I have any say in this matter to make him stop doing this or am I at the mercy of his decisions since it is only on his weeks with the kids!?

Any advice would be appreciated.