people seem to conviniently forget that a unknown amount of time has passed since the last time her age was stated, all we know is that 10 years has not passed yet since she saw Lord Hell last time.
and also, this series is chinese, the age of consent in mainland china is 14 so she is well within legal age, by size he looks like a about 3, and that indeed places him within her legal are range, and besides, it's not like it's illegal to have sex when below the age of consent as long as your partner is not 14 and older or to young it's perfectly legal.
so yes, for him to be her son is actually entirely possible and legal in their world.
oh, and do keep in mind that legal and socially acceptable is 2 entirely different things.
funny how different cultures work huh? OH! and since around mid-2020's chinese lawmakers is debating whatever or not to raise the age of consent due to cases of abuse.