Summertime writing projects are a great time to work on proofreading skills. My students hate to proofread their work! I guess it's because it takes longer to finish it if they take the time to proof it. However, it is a wonderful skill to have. One thing about proofreading though -- it is really easy to read right past an error when you are the one who made the error. I am not quite sure why that is. One thing that helps with this is to use text-to-speech software to read your work back to you.
The first step is to get the software you need. You can get free text-to-speech readers online. The voices are a little mechanical sounding, but they are decent and get the job done. See Computer Assisted Reading for more information about text-to-speech.
Teach your child that the final step before calling their work complete is to have their reader read it back to them. It is important that they watch the words as the reader speaks. When they hear an error, they should stop the reader and correct it. I tell them to back up to the beginning of the sentence and have it start reading from there. That way they can make sure that the correction is what they meant to put in.
Students can catch most of their errors this way. It cannot catch homonym errors because the words sound the same, but for the most part their work will be greatly improved if they use this technique.
By the way, our teachers use this method when proofreading their reports! It works for them, too.