There are lots of great resources especially designed for kids to help make learning guitar fun and easy.
Little Kids Rock is a great site with free online guitar lessons for children. They have lessons on getting to know your guitar, tuning, and some of the chords that kids need to know to get started.
If you are looking for a bit more instruction or a more complete package course there are a few around that are good for beginners. Kid’s Guitar Course Complete (Book, Enhanced CD & DVD) is DIY course that includes a book, CD and a DVD. The goal of the DVD is to teach the basics of guitar playing to 5-9 year olds. It also contains activity worksheets that will help your children learn how to read music.
My Guitar is a DVD that has an animated character named who helps children go through over 50 lessons written by a professional instructor. It goes over all the basics, like how to hold and tune a guitar; it also shows kids how to strum chords, read tablature, and even how to play a few simple songs. The DVD includes a link to a bonus download tool kit with an automatic tuner, metronome, chord dictionary, and digital recorder.
There is a fun app called Kasey’s Guitar Jams for Kids that contains 17 play-along songs that will help teach your child how to play along with music recordings using basic music notation. This app works best if you are a parent with at a least a little bit of a musical background.