Man i swear this is like the 5th manga I read that has this theme.
I dislike it, feels dishonest, cowardly, perverse even. Worse than the alternative somehow.
You get this very young character who's like "uWu master I just started ovulating please fuck me" but the MC says "no sorry you must be at least old enough to have a magic driving license, I am a moral slave owner after all, unless you grew a pair of D cups then I guess it would be alright" and so she does. Author-sama, you control the story you create. Perhaps if you want to be moral, then maybe do not make the child cock hungry, hym? Perhaps if you want to go that path a timeskip would be more suitable instead? Timeskip would also allow for a more natural relationship development, fixing the glaring issue with the female lead who swears eternal love and devotion to the MC for a equivalent of a KFC chicken bucket.