Aren't you making an assumption that he is the 'father' of those children ? The AP says, "black father...." and leaves it to us to assume the rest. He could be the father of 10 other children, and not the ones in the car. For all we know, they weren't his and the police thought he might injure them if he got in the car. In his condition, who on earth would trust that he wouldn't ?
Blake was the father of the children in the car....but Blake had just assaulted the mother, stole the keys and was taking the children and a knife somewhere.
Police would be irresponsible to allow that to happen.