You will be able to link another child to your account if you have been given parent code for your other child.
Where do I find my parent code?
You can request the parent code from the school or simply ask your child to log in to their account on a web browser and go to Account Settings:
I have my code, how do I add another student to my account?
On the website:
Log in to your existing parent account
Click on Account Settings
Scroll down to the "Link another student to your account" section on the right and enter your parent code.
On a mobile device:
Open the menu and go to Settings
Select Student Management
Tap the ➕and enter your parent code.