Our 4 year old watches Star Trek with us, and has become a bit of a hardcore fan. He has created his own episode titles though.
Kirk and the naughty baby - Friday's Child
Kirk and the Haunted House - Catspaw
Kirk and the naughty Spock - Mirror Mirror
kirk and the rock monster - Savage Curtain
Kirk and the pizza monster - Devil in the Dark
His main observations at the moment are to do with uniforms and hairstlyes : why does Kirk wear green in this epidode? Why is Sulu in red? Why is Chekov's hair funny? Doesn't Kirk know how to use a tricorder?
Favourite eps include Catspaw, Tribbles, Gothos, Arena, Man Trap, Friday's Child, Day of the Dove, Devil, Shore Leave, Savage Curtain.
Anything with monsters and plenty of action is a winner.