First you will need to send an email invite. You can do this from the family screen. Select the family you want to send an invite for and then tap the invite button.
Enter the name and email address you wish to send the invite to. Tap Invite. Send the email.
Do you have a gmail or hotmail email address? Instead of using your childrens email addresses you can reuse your gmail/hotmail address.
If your email address is firstname.lastname@example.org use email@example.com, firstname.lastname@example.org and so on for inviting your children (they will need to login using these email addresses).
You can also use their names: email@example.com, firstname.lastname@example.org
If the invited family member already has a Chorzzz account they will need to select your family. If they didn't have a Chorzzz account one will have been created for them. Login details are in the invite email. They are also available on the family member page for that user (if you re-used your gmail/hotmail email address you will need to forward the login details email).