Try the fit in person
Have your 9-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
Choose a simple tune together and split it into phrases of just a few notes. Invite the child to write one small practice goal on a note.
Which guitar size for a 9-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.