Try the fit in person
Have your 6-year-old hold several guitars while seated. Can they reach the first frets without stretching, relax their shoulders and wrists, and keep the instrument steady? A half- or three-quarter-size label is only a starting point, not a rule.
A first activity
Turn the first notes into a listening game: ask the child to point to a high and low string, then pluck them in turn. Start with a short rhythm game rather than a long chord lesson.
Which guitar size for a 6-year-old?
It depends on the child and the model. Try different sizes and check reach, posture and how easily the strings can be pressed.