As much as possible ignore such reactions.
It is not at all unusual for young children to display sadness in the wake of such contact. It' s important that you convey to your child an understanding, supportive, uncritical stance. Your child is entitled to his/her feelings. try to arrange things in a way that minimizes demands in the immediate aftermath of contact, so that your chikld has 'permission' to experiencve the sadness at separation from his/her other parent.
Thank-you. That is helpful advice.
What about the child throwing tantrums about wanting to live with the other parent any time cosequences for unacceptable behavior occur? At home and at school.