I think that's one reason for Toji to remarry and have Mrs. Fushiguro take care of his son (one more reason is to throw away the Zenin name and free himself and Megumi from them), but Mrs. Fushiguro leaves a while later too.

While Toji was separated from his son he meets Naobito and makes a deal for Megumi's sake. He must have known that living with Mrs. Fushiguro wasn't the best choice for his son, and he promised his late wife to take care of him.

Toji probably never wanted to sell his son, but he thought that to be for the best. Also acting like an ah as to not get too emotionally attached to Megumi. Hoping maybe if Megumi thinks badly about him then he won't live a lonly life longing for his father forever.

Few years later he goes against Gojo and vets killed. All because he again, wanted to feel worthyand be accepted. He remembers his late wife and Megumi in his final moments, realizing that with her passing he lost the one single place that could have ever accepted him

But what about Megumi? Initially Toji was still okay with leaving him to the Zenins, but on a second thought, there's the strongest sorcerer standing before him. The Gojo heir. God, the Zenins will hate this, so he tells him about his son. Hoping for the best.