Im not sure if head and shoulders will be too strong. You can try something more gentle for children.
Sebamed
http://www.watsons.com.sg/Product-Categories/Hair/Shampoo/Anti-Dandruff/Anti-Dandruff-Shampoo-400ml/p/BP_82601
You can also check out this article in asiaparent for some dandruff home remedies.
https://sg.theasianparent.com/5-home-remedies-dandruff-really-work/
Read more