Make sure that you are logged out of any goggle account associated with that device, as only then can your child log in with their own credentials.
In the email box, you will need to enter your child’s username directly followed by @gordonbrock.lewisham.sch.uk, so it looks like an email address:
email@example.com (not lgflmail.net)
For children who have not access Google Classroom before, they must ‘accept’ the invitation that they have been sent to their classroom. Once accepted, they are in their classroom and will be able to view any messages that the teacher has written, and the assignments that they have been set.