J.☯.
We've always used a child's baseball bat. We try to find a way to let all the kids hit the piñata, which means you don't want it to break right away. You can go smallest to biggest. If there are a lot of kids you can blindfold them the first time or do 1 swing each the first time.
FYI, piñatas are empty when you buy them, so you do have to fill it with candy or trinkets. That might seem obvious, but I've heard stories of kids breaking an empty piñata because the adult thought they came prefilled.