A3 1589 add articles to kids

Add articles to the list of source systems that is eligible to be rendered in ActiveKids
Two questions: 1) Should kids_friendly_source_system? be a private method ?
and independent from the first question, 2) Should we test private methods ?

On the second question, I think the overwhelming answer from the software dev community is no. (If the answer is yes, how do we test private methods ? They are private. Tyler has suggested using object.send(:private_method_name) )