This might not be about food at all? Have you had a chance to have a chat alone just the two of you instead of cracking up because you have no time for such chat in the morning?
I wonder if she’s feeling left out because you are too busy with the others and as the eldest she’s getting little attention from you? She doesn’t want to grow up any more and lose the little time she has?
If she has to make her own breakfast it’s yet another thing the others get done for them but she doesn’t and ‘it’s not fair’?
Perhaps for a few days you could do it together - her doing whatever and you overseeing it completely (while you do doing nothing else)
Yes it will take way longer but she’ll feel like you are doing it together and she will get lots of praise and told how well she’s doing good job etc.
And then you can take a step back and supervise while doing other things. Giving lots of praise, good job, really helping mum, big girl, well done etc stuff. While you crack on with the others.
Yes it will take longer for a few days but the returns could be worth it.
And if that’s the case about time perhaps you could think about what you can do so she feels more included.
And if it’s not that and she gets lots of you-plus-her alone time together after school I apologise. It could just be lockdown stress.