Is it contempt if one parent does not pick up children on assigned day

It is not necessarily a “contempt” if a parent does not exercise parenting time.  However, if the parent does not present for parenting time and it costs the other parent financially (ie time off from work, child care, etc.), the parent may be able to request reimbursement for the parent’s failure to exercise.  Here, the court will look at whether the parent gave adequate notice of cancellation, etc.   That said, if a parent continues not to exercise, and there is any detriment to the child as a result, an argument may be made that the parent’s time  should be vacated.