system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do child actors never or rarely receive any top billing on movies depsite being the titled or main character?
gpt
Top billing is something negotiated by an actor's agent when making the contract with the production company.  Tom Cruise's agent can ask for top billing, and they will give it to him.  Because at that point, you either have Tom Cruise or you don't have Tom Cruise.  You can't just slug in John Travolta is Tom Cruise refuses to do the movie without top billing.  

However, with some unknown child actor where it might be their first or second movie, nobody is going to see the movie because of that specific actor.  Even if they really like the actor, the production co. can simply say "we will just choose one of the other 90 actors who performed well in the auditions and didn't demand top billing", and then agent will be like "well wait wait wait, we don't HAVE to have top billing" because an agent who blows something like that will not be an agent for long.  That's how negotiating works!.