When is the best time to have a child tested for diabetes?

Whenever needed. Good reasons for testing for diabetes vary depending upon circumstances. A child who seems to be eating a lot but losing weight, abnormally thirsty and urinating a lot, or with sudden blurred vision should be checked immediately. With risk factors for type 1 diabetes (family history, other autoimmune diseases) or type 2 diabetes (family history, obesity, dark skin patches) screening may be needed.
Whenever there are. Anytime there are symptoms suggestive of diabetis, like excessive drinking, eating, weight loss, urinating alot you need to check with your doctor about diabetis.