"लड़का एक बच्चा होता है।"
अनुवाद:A boy is a child.
"लड़का एक बच्चा होता है।" is a generally speaking format because of "होता है". The meaning is "any boy is a child" and not a specific boy. "The" is the definite article in English and would have meant a specific boy.
Had the sentence been "लड़का बच्चा है।" then you would have used "The boy is a child"