You can securely link any number of your children under the one username, here is how...
You will have received email invites from the centres asking you to link to your children. Each email will have a 'Get Started' button in it, click that button in email 1, SIGNUP with a new username and password (DONE), now go to the other emails, click the 'Get Started' button and now LINK with the username and password that you chose from step1.
This will link all kids under the one username )
Happy Kinderlooping!