I think this is a wonderful way for parents to be involved in their children's schoolwork. This, along with reading the same books, is one of the best ways a parent and child can spend quality time together. There is nothing wrong with having a student show a paper to someone else for editing. It is very difficult for anyone to find errors in writing, and it is always nice to have someone else help.
Professional writers have editors who are paid to look for errors.
Of course, students do need to learn how to edit. A good technique is for a student to edit another student's paper, and vice versa. This way, both papers get read by fresh eyes, and both students get practice at editing. For parents to help their children edit, another method should be used. Follow along, and make sure that every pause, emphasis, and inflection is reflected by the appropriate punctuation mark.