My son petitioned for joint custody of his daughter last year. His ex-girlfriend and him were awarded joint custody. They share rotating days off since they both work the midnight. We (the grandparents) keep his daughter and her son on the days and nights they have to work. My son's ex have just filed for modification of custody and now wants primary custody and child support for their daughter. Is this possible since they both still work midnight shift and we have provided childcare for both children.