Eating foods high in carbs, like cereal, bread and other sweets is a bad idea at night because your body most likely is not going to get a chance to burn off this source of energy, and that energy (carbs) is going to be stored as fat. Once stored as fat, it's a lot more difficult for you to break down that fat and burn it off.
For an alternative to cereal as a late night snack, I would have something high in protein with some good fat (EFA's). Celery and peanut butter, cottage cheese and peanut butter, or a protein drink with some fish oil capsules are all good choices.
The theory behind eating something high in protein with some good fat (EFA's) in the evening or at night is that your body will switch over to using fat for fuel instead of carbs and that your body will break down some of the stored fat on your body for fuel over night. Ideally your body will continue to use fat for fuel until the next time you have carbs.
The protein in you night meal will help you muscles recover, and that muscle will help keep you metabolism boosted further helping your body burn fat.