Parents are using the multiple registration option to register their kids are not receiving their childrens' login credentials. The children do not have an email so no email address is entered for them.
When someone is registered as a secondary registrant, an autoresponder email is sent to them with a link to login. However, by default an email address is not required for a secondary registrant so if no email address is entered the autoresponder will never send. If children are going need to access to login (or the parents will want to log in as the children), the parents will need to enter an email address (the same as the email they entered for themselves or a different one) for each child they are registering.
If you would like to enforce email addresses be required for secondary registrations, contact support to have that turned on.