Photographing a child playing baseball can be both fun and challenging. It's important to include action shots as well as some candid photos of the child's face. It helps to have a long lens such as a 200 mm so you can get a fairly close up photo of the child playing in the field. When photographing some action, make sure the shutter speed is fast enough to freeze the action such as running. Mostly, have fun and experiment around with your settings.