Eating cereal at night before bed is not always a bad thing unless you eat cereal with high sugar content which could cause you to stay awake at night.
The high sugar content can also sometimes wreak havoc with your blood sugar levels so if you have high blood sugar then I would avoid eating cereal at night if it has a high sugar content.
But just eating a bowl of cereal at night before bed is okay and should do you no harm.
I sometimes eat a bowl of cereal in bed before going to sleep while I'm on my laptop writing some emails or viewing a few websites.
Then after eating the cereal I'm done with my laptop and put it away and then 10 to 30 minutes later I'm ready to go to sleep.
Having something in my stomach at night before bed helps me sleep much better.