Well I can say your 8 y.o. sounds like my 8 y.o.
She leaves the bathroom light on, doesn't flush the toilet (and dribbles on the seat), leaves the yogurt cup and spoon on the floor where she was sitting, leaves the back door wide open when she lets the dog in. One thing that drove me nuts for a long time was that she'd take her shoes and socks off immediatly upon walking in the door from school. And I mean immediately, with little brother and me standing right behind her trying to go around her. She's not so bad at this anymore.
Ds, almost 4 years old, is in parent participation preschool. So I'm in his class one day a week. It's been educational, to say the least. I found that my son is quite normal and that yes, there is the occasional kid who's physical dexterity and ability to focus on the task at hand are above average. And I tend to focus on that exceptional child and wonder why my kid isn't as talented.
: Only for a while. Then the extra talented kid demonstrates some weird, unique obsessive behavior and I'm back to appreciating my own for who he is!
Sorry, got of topic there.
Anyway, sounds like your kids are normal. Doesn't mean it doesn't bug you!