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.
Electric tuners are easy to use and very accurate. Hold it to the guitar and pluck the high E. The tuner will tell you if the guitar is "sharp" (too high) or "flat" (too low). Pick each note and tighten the string to make it go higher, or give it some slack to lower it. Make sure the room is quiet when using a tuner because the microphone on the tuner can pick up other sounds.