Well white on white sex should be outlawed, made illegal with severe punishment for those who break this law, the only time white on white sex will be permitted is under strict control, to keep the levels of white slaves available.
The black community will not be responsible for white children, they can of course purchase them as slaves, if they choose to. Any white children already born when the BNWO takes over will be the responsibility of the white father to pay for, even if the child is sold to a black master, the white father will have to pay for the childes costs to the new black master.