Weeing constantly through the night, constantly requiring drinks, rapid weight loss, forever feeling tired and lethargic, are all the first key signs of diabetes. 
I was diagnosed with Type 2 diabetes about 5 years ago, when I was 58. Never had any of the symptoms you mention, and still don't. My blood sugar levels were controlled by diet for about 3 years, but I'm on medication now, as the levels are gradually increasing, which is normal for the disease.
I'm surprised about the blood test information appearing here - the usual diabetic monitoring test is known as "HbA1c" and doesn't require fasting. The fasting blood test is for high cholesterol levels, I think.
High cholesterol, high blood pressure and Type 2 diabetes are all associated with being overweight and/or smoking. I became very severely overweight (obese, actually) after I gave up smoking 8 years ago. The diabetes is probably hereditary - my father had it. Funnily enough, my brother, who has edited a book on diabetes and knows an awful lot about it, has managed to avoid it so far.
Take the child to the doctor - don't muck about with self testing. The doctor will be able to do an immediate test using either a urine strip or a finger prick - instant results. If the blood glucose levels are high, there will be further tests.