I am a Nanny who isn't getting paid overtime

I got a position for a family here in Ohio. They advertised through a 3rd party but still hired me in spite of dropping that 3rd party. At first I was told I would get 15 an hour. Then it suddenly changed once paperwork was being signed. I was offered 42k a year. Which to me was fair. They then started ramping up the hours I never worked less than 60 hours a week and even worked over 80 hours one week. i finally approached them about the law about overtime. I was offered a 500 dollar bonus as a "peace offering" and told that "My time was to be seen as an investment." I love the kids and I don't mind working the hours I just find it unfair that I am working that much and only being paid at the base rate. I am exhausted and they are now treating me differently due to bringing up the law. They put another nanny on my duties and took away all of my keys, and parking passes. I had to pay $25 dollars to get out of the parking lot that day. Now I have to go back tomorrow & do the same.