Toddler-inspired: If something "comes through" do you get nutritional value from it?

[MsWhatsit]

Warning: gross diaper talk ahead.

I am inspired to ask this question after changing one of Whatsit Jr.'s diapers yesterday. The contents appeared to consist entirely of undigested raisins. I mean, seriously. It looked like the kid dumped a jar of raisins down his shorts.

Upon viewing this spectacle, MrWhatsit voiced the same question that had occurred to me: "So, is there even any point in feeding him this stuff?" Seriously, it looks like it's going straight through. Maybe not every single raisin, but a lot. Also happens with other food items sometimes.

Also, why do babies/toddlers experience this phenomenon, but not adults? I could eat like a pound of raisins and they still wouldn't appear magically undigested on the "other end."

And now, having run out of semi-clever euphemisms, I throw the discussion over to y'all.