You can not violate the custody agreement.  You can only apply to the court for modification based upon "the best interests of the child" and maybe even on an emergency basis. Although I can totally understand your feelings here it could be that she is getting the lice from school - which is generally where it comes from - and not from the other home.  An investigation would have to be had to make that determination. You may have to show that it is neglect in some way here.  Please seek legal counsel.  Good luck.