People who make good television.
Parents whose encouragement of their children takes unusual forms. Who push them in ways that verge on abusive. Who are happy to throw words like 'genius' around.
Families with an interesting backstory. This could be an immigration story or a story about child who is being encouraged to step into she shoes of a family member.
Unusual children. Children who like to talk a great deal and perform. Children who are unusually dressed. (Small children in the kind of formal wear that very few adults choose to put on.) Children who are so painfully introverted that speaking to a camera causes them difficulty, and who are frequently on the verge of tears.
People who don't make good television.
Relatively ordinary families who may have a bright child, but don't feel quantities of public exposure will necessary benefit that child - and who are likely to have withheld consent for more than minimal filming if that child has put themselves forward for an event that's being televised. People who are measured and reserved.