@Serverclosed, oh you and your stereotypical thinking... Which 90 y.o. woman do you imagine? Is she looks and behaves like our FL? Or your typical grandma? And why do you "playing" with age gap that way?
Why not ADD 20 to 30 years for each? 120 - 130 y.o. with 30 - 40 y.o., still wrong? Or, like, people can't live so long?
Your arguments are flawed, 'cause you keep forgetting one crucial thing: this novel is in fantasy setting, our common "earthly" morals can't be applied there.
So it might be "morally dangerous right there and on the long term" if: this was in our reality, FL was real mother of ML; FL was 90 y.o. grandma and our ML was common 9 y.o. kid...
My point here: stop mixing fiction and reality, relax and enjoy. Or, if you can't get over your fixation on reality, just go on.