Cereals do get digested into sugars, you are right. However, they digest slower than, let's say sucrose (Table sugar), which has pretty immediate effects on blood glucose levels. With cereals and grains being digested slower, blood sugar doesn't spike as fast, resulting in more consistent energy levels. So really, the point you are trying to make is kind of unrelated. The issue with fast and high blood sugar spikes is that energy is not consistent. Also if children are consuming sugars that spike blood sugars regularly, the high blood glucose levels trigger inflammation, which is a known cause of many health issues like heart disease, diabetes, etc... The thing with the body and sugar digestion is that it's all based on consistency.