Cereal is not good for bunnies. Just like us, rabbits love unhealthy treats. Unfortunately, unlike us, they have an extremely delicate intestinal bacterial flora which easily can be damaged by too much starch, fat etc. Rabbits need a diet very low in nutrition – that’s how come they eat (and should eat) almost all the time, preferably hay and grasses. That’s the way they were made by nature.
They will still come begging for peanuts, jellybeans and cookies though. Don’t give them any on a regular basis. If a bunny gets tummy-upset it can be very troublesome and may even lead to very serious complications.
That said, some bunnies are more sensitive than others. Not all rabbits drop dead from a few pieces of cereal, far from it. But since it’s genuinely unhealthy we still don’t want to feed it – I’m guessing you want only the best for your bunny since you took the time and effort to write here and ask!