Well like she said, she's technically not a child so I don't see anything wrong with it, like people don't like a 30yo in 15yo body MC liking 15yo FL so this shouldn't be an issue unless you want to contradict your own logic.
Editor: Hey, so the MC knows the hero is a woman… Mangaka: Yeah, I thought readers would get tired of a gag of him being the only person not to know. Would be really hard to keep it plausible, y'know? Editor: But if he knows she is a woman, than what's stopping them from getting together? Mangaka: Oh, uh… Well, you see… It's because… she's… actually only 5 years old! Yeah!
If you guys think it's bad, neptheros from archdemon dilemma is also a homonculus, a clone to be precise, who's having a relationship with a mage while she herself is no older than a year so.....😬