I am not an attorney and cannot advise you about which information to include on the supplemental petition for modification. However, in general, as you stated yourself, the fact that the children are staying with you on different days than the days set forth in the parenting plan may be enough substantial change for the basis of your modification. Stated on the instructions for the Supplemental Petition for Modification of Parenting Plan 12.904(a):
"This form should be used when you are asking the court to change the current parental responsibility, visitation, and/or Parenting Plan/time-sharing schedule. A determination of parental responsibility, a Parenting Plan and a time-sharing schedule may not be modified without a showing of a substantial, material, and unanticipated change in circumstances and a determination that the modification is in the best interests of the child(ren)."
