Size Guide
If you normally buy by age, the following may help.
Chest 28” usually fits age 9/10
Chest 30” usually fits age 10/11
Chest 32” usually fits age 11/12
Chest 34” usually fits age 12/13
Sleeve lengths are usually quite long. If the body fits but sleeves are too long, just turn under and tack down.
As a rule of thumb, buy a size smaller blazer to pullover, ie 30” blazer usually goes with 32” pullover etc.