A child as young as 4 can begin to learn if they are really motivated and have a patient, creative, and devoted teacher. However, we've found almost all kids under six are too young to benefit from formal guitar lessons, as they require dexterity and levels of concentration children their age can't provide. That being said, lessons do not need to be formal right off the bat, and can just provide a fun introduction. Also, your child may benefit from using a ukulele to start, as it's smaller and easier to learn, but very similar instrument.
There's an abundance of guitar information out there on the web, some good, some not. I stumbled across Justin Sandercoe's site a year ago and now tell everyone about it. The lessons are conveyed so clearly, concisely and in the most congenial way. The site is laid out logically as well so you can to go straight to your area of interest... beginner, blues, rock, folk, jazz, rhythm, fingerpicking... it's all there and more. Spend ten minutes with Justin and you'll not only play better but feel better too. From novice to know-it-all, everyone will learn something from Sandercoe.
The simplest answer and the one that no student that ever wants to hear is practice. Changing chords is the process where many beginners fail, and quit. But after that, the rewards will be simply impressive. There are a few tricks to get a chord transition to happen faster. Use a metronome: Set it on four beats and set it as fast or as slow as you want. Then get a chord in your mind, say D. When the metronome reaches its last beat, press down the strings. When it happens again, strum it and let it free. Then again. Do this 10 to 20 minutes a day and in less than a week, the chord progression will begin to sound much better.
Beginning Guitar Class - Open to all beginners age 13 and up, or intermediate players who want the review. Begins with holding and tuning your guitar, names of guitar parts, and quickly progresses to chords, scales, tabulature, and playing basic songs. A variety of styles are introduced to encourage students to explore and find the styles they like best. Classes are organized according to age and skill level, so it's an easy way to make new friends too. Classes start every month and may be repeated as often as you like. Cost is $70 per month (4 hours).