Kind of a strange word to teach so early in the tree... it's not even a Ukrainian dish.
The most common way of saying this in English is cereal, which generally - although not necessarily - refers to breakfast cereal. I'm not even familiar with the word 'muesli' in English.
All of my Ukrainian friends here in Ukraine use muesli for breakfast cereal. I understand that's anecdotal, but i think the program should accept cereal for мюслі as it is a pretty common colloquial translation.
Cereal for breakfast = кашу на сніданок
If by "cereal" you mean "oatmeal" you can say вівсянку
If you mean "corn flakes" you can say кукурудзяні пластівці
Somehow people use "cereal" to only mean "breakfast cereal". Cereal has a much wider meaning than that, buckwheat and barley and millet are cereals as well, it's just that people usually don't eat them in the morninng.