Can a parent/legal guardian sign a contract for a minor?

If I decide to open a Cheer Gym; I would like to have the option to legally bind an athlete to our gym for the season. I know this looks wrong, and sounds wrong, but there are many events in the season, and having athletes decide they no longer wish to partake in anything to do with the gym is a difficult thing to deal with. I have heard stories of athletes leaving/switching gyms just weeks or even days before huge event, and that causes a lot of stress for not only the gym owner, but for the coach, and for other athletes as well. The contract wouldn't strip them of their right to walk away, but would only incur them a fine if they decide to. That being said, the Cheer industry is one for the young, and most participants are under the age of 18. The big question is, can I have the child's parent/guardian sign the contract on behalf of the child in question? Mind you, it is not definite that I will end up opening the gym. I might do it someday when I am in my late 30s or early 40s, but definitely not now.