Visiting the park is a joy...you see the fearless try to fly and the nervous take small steps...but the kids just know to keep moving. If you stop moving of course parents may just find you and take you home!
For this one I framed and shot the photo a little larger and cropped when I got home. I do that a lot when photographing action. You just have to know where you put your focus point, and make sure that was actually the part of the photo you wanted attention drawn to.
Some people do focus, recompose and then shoot. This is ok if you are using a really large depth of field, but will not work on shallow depths. The focal plane will just be too small, and your properly composed shot will be out of focus on the heart of your photo.
So the moral of the story is...frame enough to catch the action and then use your little select autofocus point option to get the part of the photo you want sharp.