if mom moves and now whats to change drop off and pick up location and times what should be done when dad cannot do what the mom wants and now is

saying if you don''t do it I will call the cops. What happens when the mom doesn''t show up to get the children and dad will not give in to the demands with out her going to court to modify the agreement in place. Mom didn''t get the children at pick up place and time and now the children are still with dad It is his weekend but it is only Thursday so now dad will have children from Wed to Sun when the mom is suposed to pick up. What happens if she doesn''t show up again?