Dоg Trаining: Whаt shоuld I dо if my dоg wоn't stор bаrking?

When our dogs were still puppies, one of them has that habit of incessant barking. Barbie is a pekingese which is a lap dog. And when she is left alone in the living room, she would bark until someone comes to her. We have corrected that problem by putting her in the cage whenever she would bark. On the 5th occasion, her barking was subdued until she didn't bark anymore. But now she is turning 4, her barking issue is when she hears children's voices (those passing by in the street). We cannot control her barking for now.