Are you dependent on your parents at all? (Ex: you are a child, or they give you money to live.) Send it. You don't have to mean it.
Otherwise, do what you want. Don't send one if you don't want to.
A lot of us had hard lives and didn't abuse children. Ultimately, we are responsible for our own actions.
As a side note, if he cannot stop abusing children, is there any way you can get them out of that situation, such as reporting to child services?