The real question to ask is how fresh is the fish?
The risk of eating raw fish because of the parasitic dangers that lurk within uncooked meat applies to both adult and child. A child with a weaker immune may find it harder to stomach the bugs (pun intended) and suffer food poisoning.
Raw fish should be of sashimi grade, sold in clean and hygienic premise. Ask about the frequency the fish is flown in. Daily is great.
Most parents are comfortable to let their child try raw fish between 3 to 6 years old. The tip is to start slow in small quantity.