Quick, cheap non-invasive way to get good indicator on your suspicion is simply buy the old classic urine test sticks at the pharmacy and check it out.
Weight loss is indeed often the first symptom of diabetes in small children. Small kids who have type 1 diabetes will be eating regularly, maybe even more than usual, but will be unable to gain weight. Unexplained vomiting may also appear next, aka the kid may throw up over a three- or four-day period for no apparent reason.
And as you are a diabetic yourself, the ultimate yet simple is drawing a small blood sample to check your child’s blood sugar level. Just do a quick finger or heel stick. A reading on 11-12 mmol/L or higher would mean the young child could have diabetes.
The health of your child is in any case much more important than what your doctor may think about you overreacting. I am actually sure he/she will be very understanding of your concern. So do never hesitate. The doctor can also get the A1c test done.