If one parent thinks the other is truly "unfit" then the Dep. of Fam. and Child Services should be contacted. This issue at this particular stage is not a legal problem, it is a family matter. You mentioned that the couple is still married but living apart, so at this juncture there is not a custody issue because custody of children is ruled upon by the court after a divorce proceeding. I would not recommend either parent taking the child hostage and further, the situation of "private property" really has no significance here. Bottom line, try to persuade DFCS to intervene on your behalf.
Well, to begin with, two wrongs do not make a right. I do not think it would look very good in court if the mother said the father refused to hand the child over, whether it is on private property or not.
Every father has the right to see his children. If I were the father I would talk to the attorney to see if there is anything that can be done in the meantime, or even see another attorney. It is not legal for a woman to withhold a child from the father, just as it is not legal for the father to do the same.
When two people fight over custody the only individual who really ends up getting hurt is the child. Many times parents will fight over the children just to get back at one another and that is a shame.
I posted a website below that discusses South Carolina law regarding children, fathers rights, and divorce.