Get her a scoop for the cereal? So she can have one scoop, and do it herself. And if she eats all that, then she can have more/ two scoops the next day. That's how I'd play it. And if she only eats some of her one scoop, then no big deal, it's minimising waste.

My dd does this with drinks, but we just put them in the fridge and she can keep going back to the same one.