For you guys who are confused :-
मैं उनका बेटा हूँ MEANS I Am His Son(formal = when you respecting your parents, and It's the right way )
I Am His Son(मैं उसका बेटा हूँ), means the same tho.
But you should go for it
Can बेटा also mean “child”?
No, child means "बच्चा"
can 'unka' in this context also mean 'his' or 'her' ?
Yes, it can.
Is मैं उसका बेटा हूँ an incorrect sentence?
I think it's correct. It would mean I'm his son or I'm her son.
As AmiSUBHAM implies - it's correct but disrespectful. Like saying तुम rather than आप.
I find it confusing when one of their drawn female characters says a sentence like "I am his/her son"...
Mind you it could be a son with long hair... I think it does not help understanding context though.