I don't think "who did it" matters overmuch. What matters is you have a bowl of cereal on the table. You need to determine why that's a problem for you - is it the waste, or is it the dirty dish not cleaned up?
If the waste is an issue, make it clear you're only buying two boxes of cereal per week, and they're welcome to eat them or waste them, but you're not buying more. Let them police it amongst themselves if one of them is consistently wasting cereal. Eventually they'll run out of cereal and be hungry, right? Lesson learned.
If the dirty dish not cleaned up is a problem, then again - let them police it. The dish needs to be cleaned up before anyone has screen time. You don't care who does it - but it needs to be done.
Now, since you have such a diversity in ages, it's possible you'll need to work with the six year old a bit to make sure they're not constantly relying on their siblings to clean up after them; though that's not a bad lesson for the older ones I imagine (they will need to learn how to gain his/her cooperation). But it doesn't necessarily put the six year old in a good situation, so you may want to work with them.
But not when it's unclear who's at fault; just make sure to pay attention to their dish-clearing or cereal-wasting habits directly, and if you don't see a bad pattern then it's not an issue - and if there is an issue, you'll see it if you're paying any attention at all, even if you don't see every instance.
All in all, the point is that looking to assign blame is not the point, and you shouldn't do it. Look to make sure your children individually know what they should be doing and usually do it, and make sure that they as a group take care of the problems that arise (messes, waste, etc.), and you'll be fine. Focusing on blame is what leads to the lying - don't put them in a situation where they feel like they need to lie, and they won't (and hopefully won't develop a pattern of doing so).