I think when kids are younger (how young? I'm not clear) they simply sometimes don't chew their food thoroughly enough. I saw chunks of raw carrot and pumpkin seeds in my son's diaper for quite a while, and I think it was an issue of chewing. But, I'm not 100% sure--I don't think he chews _that_ much worse than me (he got teeth early, so it's not an issue of lack of teeth) and I don't see undigested raw carrot or pumpkin seeds. Part of me wonders if there is a natural progression of how well their digestive systems break down foods, as part of changing from an infant to a toddler to a little kid.
And I see more undigested stuff when they've recently been sick (and floating poop, so I think fat digestion gets worse then as well, but just transiently).
But, I don't know the cut-off age at which you say 90% of all food should be just fine, and then what's the 100% line?
eta: in case it wasn't obvious, I'm joining the thread to learn what's normal and what's not. I know what I see with my kids, but even then, I'm really not sure what's healthy.