I'm still not understanding exactly what the problem is, but another option is to drag the food into their inventory, and then command them to eat it from there. I'm pretty sure they eat autonomously from their inventory - they do it with fruit.
Another idea just dawned on me too: if another sim is trying to eat it, they will not be able to. If you have your heart set on that particular plate of food, like if it is Ambrosia, then you will need to go into buy mode with MOO and move it so that the offending sim can't get to it. Then go out, they have their tanty, then drag it back to where your sim can eat it.