The RFU have a guide and video on how to update a youth registration on their age-grade page which you can refer to in addition to the guide below
The steps to register an age-grade player are, log into GMS and from the Summary tab – click ‘Update Family Registration’
The Registration Summary page will open, for each child you wish to update click More -> Player Registration
This will bring up a view which will ask you to confirm the player’s details, school, photo-consent, medical info etc. When finished click Next.
Having completed the information you will return to the summary view. Repeat for each child and when finished click Confirm, which will create a registration request.
Before I can accept your registration – GMS requires that you have confirmed your email address (you should have been sent a link when you created your account or logged in). Once your registration has been accepted (by me) then you will receive an email.