Um for children, Coco-Pops, Rice Krispies or Frosties.
For adults, Special K, Weetabix, Shredded Wheat, Cornflakes.
If you google Kellogs, then you should be able to track down the British versions of cereals. We tend not to have the really highly coloured and extra sugary ones that are in the States. (Or perhaps I'm just very mean and don't buy them.)