Here is something I read that makes me follow my doctors advice and tell other parents who think "he/she looks fine, suck it up": one study documented 9% of people who suffered a 2nd concussion while recovering from a concussion DIED. I wouldnt take that chance with ANY child.
Actually, the mortality rate for SIS (Second Impact Syndrome), which occurs when a person has yet to recover from the 1st concussion and then suffers a second concussion is roughly 50%. https://www.sciencedirect.com/topics/neuroscience/second-impact-syndrome