Stale Cereal is safe to eat and the only thing that will happen when you do eat stale cereal is that the stale cereal will not taste as good as when fresh.
Eating Stale Cereal is okay and poses no healthy safety risk but you might not enjoy it as much.
I've eaten stale cereal before when I was broke and had not much money to buy groceries and so I was left with eating the stale cereal I had left in my pantry.
I never got sick from it and it's okay to eat it if you want to eat the stale cereal or need to eat it.
The cereal is best eaten when it's not gone stale and while it's still fresh for the best taste and enjoyment but the stale cereal is not harmful to your health if you eat it.