I understand there has been an issue with the new look School Money system. I have queried this with School Money and the response was as follows:
Parents don't need a user name -- just their mobile number, email address, password, and child's first name.
If they're being asked for a user name, it will be because they have not clicked on SchoolMoney Parent Login. (And are getting through to the place where you would log in as a school).
Please make sure they click on the SchoolMoney parent login option.
I hope this helps. If you have any further questions, please do not hesitate to get in touch.