love child

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A child born of parents who are not married to each other.

from The Century Dictionary.

• noun A child of illicit love; a bastard.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun euphemistic A child born as a result of a romantic liaison between parents not married to one another; an illegitimate child.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the illegitimate offspring of unmarried parents