As everyone else is saying, if the contract is with underaged people then get a parent/guardians co-signature, or simply do not deal with them.

I normally physically sign a contract with a primative tool called a pen, this is a writing implement that was in great use before the rise of the keyboard. It has sadly fallen into the place that most obsolete objects go, the trash can.

Seriously though, get a contract drafted, check out my sample one in the stickied thread at the top of this forum, get a lawyer to briefly look over it (might cost $200-$300 or so, but it's worth it and long lasting). Print it out twice (copy for you, copy for them), sign both of them and send them (or fax) to your client. Get your client to sign both copies and return one to you (via fax or a pre paid envelope). Done!

The one or two dollars I spend in postage/printing is more than worth the security I have with clients. Plus I also have a physical address or an actual fax number, which can be traced if I need to send debt collectors (never happened yet )