What are things like Bread, Rice and Cereal collectively known as?
There are several terms which you can use, depending on the context of writing (or speaking). 

A very simple one is "the starch". It is mostly used in the context of meal planning, such as "What starch are we going to serve tonight" or "When planning a vegetarian meal, it is best to first decide on the starch and then select sides that complement it". 

"Grains" or, mostly interchangeably, "cereals" is what academic specialists for nutrition and diets use in their jargon. If you read a textbook on nutrition, that's where you will find breads, etc. There, the context makes it clear that the word doesn't mean simply uncooked kernels. The nonacademic literature on dieting is more likely to use "carbs" - see Chris H's answer for more detail on that usage. 

In legal language, for example rules and regulations about food product labelling, or import and export regulations, you will frequently find phrases like "grain products". 

Since none of these terms is unambiguous, outside of these genres of writing you will probably have to go for something more descriptive, for example "foods made from grains" in a colloquial conversation.