First we need:
1. to see the iWatch serial number and compare it to the invoice.
2. we must verify the invoice with your full name on it.
3. You must provide an official government picture id proving that the invoice is yours
4. Last and most important we must compare your government picture id with you live, face to face to ensure that the id is really yours.
Do you agree that to accomplish step 4 that we have a immense problem?
If I recall, your child may have entered a password and does not remember it.
We suggest that you take the watch to an Apple Store, they will be able to verify you identity and perhaps help you to reset it for a charge.