In the modern world, kids are exposed to advertisements from a very young age on, often before they can even tell the difference between commercials and regular entertainment. Once familiar with certain brands, they are typically known to have a very skeptical attitude towards store brand (also known as off-brand) products in general. Breakfast cereal is a particularly contentious subject where kids can turn from skeptical to hostile.
The main argument for store brand cereal from the parent perspective is the price - knockoff products usually are available at half the cost of the originals, or even cheaper. The main argument against store brand cereal for kids, besides the less attractive packaging, is of course mostly the taste. The classic counter-argument of the parents here would be that the taste is actually pretty much the same, if it wasn't for the branding.
The only way to eliminate most of the effects of branding are blind tests. I have been able to find a number of blind tests with different kinds of cereals on the internet, but they usually have small sample sizes - typically below 20 people. Yet the results seem pretty consistent: There is only a very small majority of kids who prefers brand cereal if they don't know whether they are eating the original or the knock-off. And almost the same amount of kids will either say that the store brand tastes just as good, or even better.
What to make of the blind tests? Even the slight preference vs. store brand cereal in the blind tests could still be a result of branding: Kids that are very familiar with the cereal brands will be able to recognize them through the taste, and automatically associate the taste with the branding. Arguably then, taste preference in fact seems to not depend on original vs. store brand at all.
However the power of branding is strong. In posts such as this, parents claiming the taste to be the same (or very similar) to their kids admit to barely being able to stomach the store brand knock-offs themselves. My suggestion, given that your kids are not used to the store brand, and given that you have chosen to mostly feed your kids sugar for breakfast, is to be sneaky: Get a large plastic container and put in the branded cereal, which has the bonus of keeping the cereal fresh longer. Get groceries without your kids, and once the container is getting empty, either replace it with store brand cereal, or start by mixing both - and hope your kids won't see through your dirty maneuvers.