The young boy was struck by a car in Lisburn, County Antrim.
The collision happened on Railway Street on Wednesday.
Paramedics treated them at the scene before rushing them at high speed the eight miles to the nearest accident and emergency hospital in west Belfast.
According to the PSNI, a second boy is in a stable condition.
The children are being treated in the under pressure Royal Victoria Hospital.
Anyone who witnessed the crash or has any information is asked to contact police in Lisburn on 101.