Step 1 - You will receive 2 emails from Microsoft Invitation on behalf of ...(to signup on Microsoft Teams) and Microsoft Teams (to join your team)
Step 2 - Sign up on Microsoft Teams
NOTE : Please note that the password will be used by your child to logon to Teams, please DO NOT use your personal email password  
Step 3 - Close this window after Signing up
Step 4 - Open the email from Microsoft Teams to join a team
Note: you can either use the Teams app or the Web version to access Teams
If you see the screen below, Congratulations ! you are all set 
Instructions to create an account on Microsoft Teams
Note: you might need to check your junk, trash or unread email if you don't see it in the inbox.
Click the 'Accept Invitation' button.
You will be asked to create an account if you do not have an account on Teams
Review and Accept permissions to complete the Teams Signup