How do i obtain medical records of my deceased child?

I'm so sorry to hear about your loss.

All you need do really is ask, and the Covered Entity needs to give you a copy. If you were the guardian at time of death, that guardianship survives.

The doctor is entitled to ask for that request in writing. Also, they are allowed to bill a "reasonable amount" to defray the cost of copying. That cost is not intended or allowed to prevent you from obtaining those records.