If you have decided that you want to fire your babysitter, here is a template you can use to fire a babysitter politely.
We thank you for your time and effort in taking care of our children. Unfortunately, as painful as it sounds, we have chosen to move in a different direction. Thus, our working relationship ends here.
This is not a knock on your babysitting abilities. We think you are great with kids. Hence, continue to pursue what you love doing and I am confident it will be a big help in the future.
We have other reasons for moving forward with another babysitter. There are a lot of factors that affect our decision and you are not one of them.