When a child signs up for an eCard, do they have access to only children's materials?
Libby has a kids landing page: https://queenslibrary.overdrive.com/library/kids that you can bookmark for your child. However, there is nothing to stop them from clicking Back to Main Collection, which brings them to adult content.
Sora is a similar product (also by Overdrive) that is geared to children. You might check with your school to see if they subscribe to that, as it is a school and home based product.
With Boundless you can create a Kid Profile (on your account) that will display digital content with Kid audience levels.