I'm going to encourage my fiance to begin introducing food to his children that their mother believes they are sensitive to and not tell her
Why would you think your fiance would do that?
Because I have powers of manipulation over people through the little aggression and anger.
Wanting to give the children normal food seems like it might be fine to you, but it can make them sick if you give them the wrong foods. You're not their mother, and you should instead share your thoughts with her if you want to change her mind.
I'm their father's wife and my personal opinion is that the children deserve better options for food than vegan.
That may be your opinion, but you're not their mother; you didn't give birth to them. It's wrong to manipulate your spouse and trick their children's mother. Just talk to her.