Your child's teacher will need to send you a print invitation (pdf) with an invite code. Login to your Knowledgehook parent account, click on "Add Child", and put in the code.
NOTE: If you have an existing parent account and your child's teacher invites you via an email link to add another child (or for the same child but a new grade) you need to first login to your parent account before clicking on the "Sign up!" link. Otherwise you will need to request a PDF version of the invite in order to get an invite code and follow the directions above.