A picture would have been helpful but it seems like your kid has got a patch of Tinea Capitis
,which is the fungal infection
of the scalp really.
I would advise to apply Lamisil
ointment on the patch 2 times a day and use NIZORAL
AD shampoo to wash the while scalp with.
It will take 2-3 weeks to completely heal up but if you observe that the medications are not responding in a week time then you need to see a dermatologist
to have a look and then proceed.
A naevus sebaceous would be a soft yellow-orange hairless patch which is not fitting into the description but as I said without having a look no confirm diagnosis can be made.
All the best.