Why didn't Baba tell the boys that Hassan was Baba's son?

Hassan was a Hazara. Traditions ran deep in this community and admitting this would have opened the flood gates for all kinds of socio-cultural and stigma pressures that Baba was not prepared to deal with.

Not also that, but a variety of reasons. For instance, he had an affair with Sanouber who was married and conceived Hassan. Baba was afraid of the sin committed, lying, stealing a mans wife, which he did.