How many single hairs can be found on a child's head?

100,000-150,000

"You are born with about 100,000 scalp hair follicles to start with, but it varies with natural hair color. Blondes have an average of 150,000 hairs, the average redhead has 90,000. Those with black or brown hair average 110,000 to 100,000 hairs.

There are about 5 million hair follicles on the adult human body and men have a few hundred thousand more than women. These follicles are distributed fairly evenly throughout the body, with the exception of our hairless palms and soles."

https://baumanmedical.com/qa/many-hairs-human-head/

Thank you, now I only to convince my son that there are that much..