I, too, am sort of self taught...my Mom 'taught' me using the 'Here, let me do that." method.
My niece and nephew, 7 and 8, visited over Easter and each made a 4 patch quilt with me. They were very comfortable with the sewing machine and thrilled to be able to use it. We worked on straight lines, they practiced on scraps until they felt like they could sew a straight line. I had to do some straightening, but was surprised at how little of that I actually had to do. Here's the post I put on QB about it.
http://www.quiltingboard.com/t-119952-1.htm
They were so eager to learn and very happy to have my undivided attention. Kindness and patience are the keys to successfully teaching sewing. And the kids have to know that there work will not be perfect until they have practiced sewing for awhile. I made some 'mistakes' while I was showing them how to use the machine, so we fixed them together.
By all means get a book to guide you, and start with a simple project. I think I need to take my own advice because my grandson wants to make a quilt this summer. The book will probably teach me as much as it teaches him!