The etymology of love child says it derived as a polite form of "love brat" which was used around the 18th century.
My question is when two people are in love and they have a child, could you not call him/her a "love child"?
Edit: Why does it have to have the (rather negative) connotation that it is outside of marriage, when love itself is a positive emotion?
Funny enough, "love child" makes sense in Indian English, where there is a concept of "love marriage" and "arranged marriage" – as has been earlied posted.
Best Answer
You can't really change the meaning of a word to suit your taste. "Love child" refers to a child born to parents not married to each other, and that is what people take away from it. If you ignore what others think and only pay attention to how you mean a word, you will cause your listeners or readers confusion or merriment.
(Incidentally, the word "love child" carries the same stigma in some other languages as well. 愛の子 (ai no ko or "child of love") means exactly the same thing.)
EDIT
Answering OP's question about how did it come to have a negative connotation. From Wikipedia article about Legitimacy:
Illegitimate is a pejorative term, and, euphemism or no, love child ultimately inherits that meaning from its ancestor.