HOUSTON --A carjacker held up a mother at knife point in front of her children in northwest Harris County. According to the sheriff's office, the mother and children were in their driveway when a man walked up and demanded the keys. The suspect drove off, but the woman's husband followed him in another vehicle to a dead end on Derby and Hallowing Point in northwest Harris County. The man abandoned the stolen car and ran off. Deputies have not arrested the suspect yet, but they say they know his name and where he lives.