Thank you for reaching out for assistance through the Community! I recommend reviewing the priority of the contacts you listed on the registration forms. I am providing a document that provides further instructions on how to complete the contact priorities. You can review the document here. Also, make sure the phone number field is entered in the correct format.
For further reference, I recommend reviewing the article on unable to Submit.
You would want to use the Returning student form if your student move between schools or grades within the same district and the New Student form if your student is moving schools from one district to another.
As you mentioned that your children are returning students, a few common reasons that would hinder you from submitting the form can be missing contact names of guardians and their contact details or some fields such as phone numbers and Date of Birth in the wrong format.
The summary page on the form should help you identify the missing or invalid fields that would be preventing the form from submitting.