Every child who hears the Mahabharata eventually arrives at Karna. The question is always the same. Is he good or bad? And every parent answers it badly the first time, because the right answer can't be given quickly.
What Karna is, structurally
Karna is the Mahabharata's question about loyalty, birth, and the cost of being given the wrong name. He isn't the bad guy. He also isn't the good guy. He is what the Mahabharata is for. The character whose life is built to keep being thought about.
When your child eventually feels conflicted about him, that conflict is the Mahabharata working. It isn't a mistake to fix.
Where to start, where to wait
Ages 5 to 7: Karna as the boy who learned archery in secret. Karna saving the cobra arrow for a son. Karna as a friend who keeps his word to Duryodhana even when it would be easier not to. These are stories of skill and loyalty, and they work cleanly without the full tragedy.
Ages 8 to 10: Karna's birth, Karna's mother, the curse from his teacher, the wheel that gets stuck. This is when the full tragedy lands, and it lands much harder when a child has already known Karna for years as a steady, skilful presence.
Don't answer 'is he the bad one?' yet
When your 5-year-old asks if Karna is the bad one, you don't have to answer. You can say: he's the one we'll keep thinking about. That's true at every age, including yours. It's a true answer that gives the story room to keep working on the child instead of being closed off.
If you say 'yes, he's bad' you'll spend years undoing it. If you say 'no, he's good' you'll spend years explaining the rest. The 'we'll keep thinking about it' answer is the one that holds.