User blog comment:IvyBushe/Muggle-born parents and their children./@comment-1400578-20130407144209

To my knowledge, that specific scenario hasn't been addressed in canon. However, for my 2 cents, I imagine it would depend on who's being asked. Rowling stated that those who make the distinctions of "pure-blood", "half-blood", and "Muggle-born" are usually those to whom those distinctions matter. I would imagine that someone not particularly prejudiced might call that child a half-blood, since their parents were non-pureblood wizards. But to people like the Death Eaters, who see Muggle-borns as just as "bad" as Muggles, those children would be Muggle-borns too. That's just my personal interpretation.