How to update field in multiple child records when parent record field is updated?

I hope someone can help me.

I have written a function to update a field called "Organisation Type" in a module Contacts whenever a similar field in Accounts is updated. This function/workflow works fine when there is only one child record but when there are multiple contacts associated with an account only one of the contact is updated.

Here is my code;

ContactList = zoho.crm.getRelatedRecords("Contacts","Accounts",Account_ID);

Contact_ID1 = ContactList.getJSON("id");

Contact_ID = Contact_ID1.toLong();

UpdateContact2 = zoho.crm.updateRecord("Contacts",Contact_ID,{"Organisation_Type":OrgType});

How do I modify the code so that all contacts associated get updated?

( I know it is odd to have two fields in two modules - but we need the field in contacts for reporting/ email macro)

Thanks in advance

Susan