A thirteen year old can’t manage? That makes me think of Mike Mac’s famous words: “If you don’t know, let go”. A common mistake on a Wayfarer is to oversheet. Other then that, get ratchet blocks, like Colin suggests.
Get rid of the fairlead with jammer, it introduces a lot of friction, and replace it with a small block. The ratchet is then put on the thwart almost under the gunwale. From there the sheet goes to a fairlead and jammer on the thwart. This time the sheet goes through the fairlead in a straight line without friction. Here is a good example of the setup I tried to describe:
Obviously there are variations on this setup, there is no “best solution” only a solution that suits you best.
By far the best source of information is a visit to one of the many Wayfarer events where you will meat friendly and enthusiast sailors who are unstoppable once you ask anything about their boat and setup. Don’t forget to bring a camera!