WI child custody law, procedure for seeking sole custody

Custody and placement are distinct determinations. Custody deals with decision making for your children and placement is about time actually spent with the children. In this instance, I think that you are really talking about placement. There are lots of reasons why family courts change children's placement. In this instance, your husband is probably in contempt of court for failing to return your children (if the court order specifies that they should be placed with you). However, since your husband has kept the children for the past six months, the court may decide that there has been a "substantial change in circumstance" that now justifies more placement time with the father. It would probably be helpful for you to consult with a lawyer in the Milwaukee area who can further help you evaluate your options. Best of luck to you!