If the mother does not give the children to the father on his
court appointed scheduled time then she is contempt of court. If
the father does not bring the children back when the court document
specifies that he has to, then he is contempt of court. If the
father does not pick the children up on his court appointed
scheduled time, he is NOT in contempt of court. It is the father's
right, not his obligation. If the father is paying child support
then he is not obligated to do more.
To go about filing for contempt you have to have a lawyer file a
document stating one parent was in contempt of court. This costs
around $200 (in the state of Iowa at least). Once the document is
filed the parent will be served and have to appear in court stating
why the kids were not given on the scheduled time. The court does
not usually find the parent in contempt if this has happened just
once, it usually has to happen three or more times before the
parent is considered in contempt of court.