"На сніданок" means "for breakfast." "At breakfast" is "за сніданком."
Future perfect "indicates an action that is to be completed sometime prior to a future time of perspective, or an ongoing action continuing up to a future time of perspective." In the Ukrainian sentence, there is nothing indicating that the action of eating is to be completed sometime prior to a future time (no future time is specified, such as "by 10 o'clock"), or that the action will be ongoing and continue up to a future time.
We can try to approximate the meaning of the perfective aspect by translating the sentence as "She will eat it completely for breakfast," but it sounds weird.