I think you need to teach the leave command or perhaps ask her to go to her bed when your son is eating so she's not walking around him, it's so tempting for her to want to take the food from him, it probably smells great.
I would stop saying no to her personally, you need to get the leave command in before she looks like she's going to take the food and then distract her with something else.
She's still so young you should have no problems stopping this with the right commands.