Milk is not a broth even when added to cereal.
When milk is added to cereal it's still a beverage because it does not change when added to the cereal so it's still a milk beverage.
A broth is a liquid that is made through the extraction of flavors in water, through the process of long, slow boiling to extract those flavors.
Broth is mostly made from meat such as chicken, beef and sometimes vegetables to extract those flavors.
But milk is just milk regardless of what it's used in.
You could however add milk to soups to add some creaminess to it but the milk is still milk and not broth.
For liquid to be broth it must be cooked or boiled and milk does not require that so it's not broth.