Can you bring breakfast cereal into the UK?

My reading of that is that you will be okay. To be sure, you could declare it on arrival. I expect that the value is low so it is not a great risk if it is confiscated.

However, I would be in the camp of not taking it and buying it here. If there is a danger of a tantrum due to the wrong breakfast cereal then you may need to worry about many other odd things here.

Where do you come from? We may be able to warn you of other potential problem as well as breakfast cereal.

What breakfast cereal anyway? We may be able to tell you whether it is easily available here. To be very fair, some companies actually formulate products for different countries slightly differently. I have actually been inside a Nestle factory making breakfast cereals and seen slightly different products going into to boxes with the same name (but different language labels). For example, a slightly sweeter version may sell better in some countries but less well in others. Typical temperatures may also affect the formulation. However, if your children spot these differences then they have a very sensitive palate. I would expect that if the box has the right name and picture on it then you will be okay.

If they have a sensitive palate then the milk may be a concern but you cannot bring in your own milk.

You do not need to declare commercial packaged foods in small volumes for tax or for biological security (boxed breakfast cereal is cooked so won't contain viable plant seeds or animal parasites).