× For breakfast, I have some milk and cereals.
√ For breakfast, I have some milk and cereal.
Cereal is an uncountable noun, so it does NOT have a plural form. Cereal means the thing you eat for breakfast: ‘I like cereal.’ ‘She never eats cereal for breakfast.’
Cereals means ‘more than one type of cereal’: ‘I don’t like all cereals, only Corn Flakes and Frosties.’