She couldn't tell Chennchenn when he was younger that "this isn't your dad, you're a child without a dad." Because doing so would not only be too cruel for a child, Chennchenn wouldn't be able to accept it, and it would also arouse the Mo Family's suspicion. If Chennchenn didn't call Mo Beiyan his dad, wouldn't that indirectly confirm the child wasn't Mo Beiyan's?
All the initial arrangements would fall apart, deceiving the ancestors and elders, marrying with someone else's child, such situations the Mo Family would never forgive lightly.
Moreover, if the Mo Family discovered the child was Mo Nanjue's, Chennchenn would be even more unlikely to stay with them. He would certainly be taken back into the Mo Family for various reasons, and what awaited him, who knows what kind of future it would be.
