I Have A Four Year Old Granddaughter That Is Extremely

I have a four year old granddaughter that is extremely over weight, and eats most anything she wants & at anytime. I believe she weighs about what a twelve year old weighs! Now she is starting to drink alot, and we would like to check her with my husbands meter, but we do not know what range she should be in? We thought this might be a wake up call to her parents.