When adding a junior to a membership package you will also need to add the parent / guardian details.
To add a member, select the membership module on the left hand side of your admin page, select the membership you would like to add the member to.
Select 'Member options' and 'Add member':
You will firstly need to add the Parent/Guardian details. If they are an existing contact then you can search for the name and the details will generate from the contact record. If they are a new contact then you will need to complete required fields.
You can then enter the juniors information, again if they are already set up as a contact then you can search and select for their details.
Make sure you use the 'save' button at the bottom of the page to add the junior member.