When your child is under the age of 13 and you use Family Link to manage their Google account, they will not be able to download Arduino Create through Google Play store. The download button will be greyed out an unavailable.
To let your child use Arduino Create Chrome App, you need to enrol them using the Chrome App for Education plan instead. This way, you will manage their account and be able to access the same app as they do. You can follow the tutorial on how to subscribe to Chrome app for Education and chose to subscribe to 1 single user for $0.99 per month.
In the fields where you are asked to enter the School information, you enter your personal information instead. You can still subscribe to the educational app even though you are not an educational institution.
Please note that after enrolling the child/student, they need to download Arduino Create for Education which is available for Google Family Link minors.
By using Arduino Create for Education to enrol your minor, you will also have access to the app.