I am not sure where you are looking, but pretty much every airline does, w the exception of intra n America fares (and well, Etihad now).
Do a matrix search for HKG-SIN on ITA Matrix for 1 adult and 1 child. Pretty much every result will come back w an asterisk next to it, which if you click thru will show some level of child discount. For business class and higher, 20-25% discount really adds up when you are bringing 3 kids along!