Amnesia is not a specific diagnosis or medical condition, it’s a vague term describing a very wide variety of disturbances of memory. If you want to study memory loss in the pediatric population, you’d need to focus on diseases or diagnoses associated with the specific type of memory problem you want to know about. You’d need to look at such things as seizures (and seizures come in a variety of types each with a variety of causes, or no discernible cause), head trauma/concussion, stroke, migraine, psychological trauma/PTSD, brain infections, diseases of the neuron itself, etc. etc. etc.
So read this chapter and read the references and use search terms like “developmental amnesias” and “acquired amnesias of childhood” as search terms in Google and Google Scholar. When you use Google Scholar read the list of “Cited By” and “Related Articles”
You seem to mean a complete inability to form memories. My first question would be “does that exist outside of movies?” And if it was genetic. complete and active from birth it would likely be labeled as some sort of acute developmental delay: that kid wouldn’t be able to learn anything.