Hello Kezia!
"Había un chico. El chico era Bart. Bart ESTABA en Barstow, California. Bart no ESTABA contento."
Your last two sentences are not correct because you changed the verb ESTAR for the verb SER.
Bart está en ... (present: Bart is in ... ) = Bart estaba en ... (imperfect tense: Bart was in ...) here you are talking about places.
Bart está contento ( present: Bart is happy ) = Bart estaba contento (imperfect tense: Bart was happy) here you are talking about emotional states.
In English the same verb is used for both sentences, but not in Spanish.
The verb SER is used in Spanish in the following situations: physical appearance or inherent personality traits, identity or permanent features (nationality, creed, etc.), family or personal relationships, when or where an event takes place, the date, seasons and time, in passive sentences, with time adverbs and other instances, such as the material, origin or ownership of an object.
The verb ESTAR is used: moods or health conditions, appearance, marital status, location, modal adverbs and other cases.
I hope my explanation was clear.
Best wishes
Sof{ia