1. If you look at kids in real life, you'll notice that they pretty much all look similar until they hit 10.
2. Good voice actors are expensive and hard to find. Good child voice actors (or women who can do male children, though it doesn't sound like that happened here) are even harder to come by.
3. I've seen different-looking kids, but just like with the advlts, some people are going to be doppelgangers.
I disagree with point 1. Kids look as different in real life from each other as advlts do.
I agree with the OP. Every kid I've seen is a clone. LITERALLY. Only one little girl had blond hair instead of brown, but her face was identical to every other female child. I thought the Jarl only had 2 kids until I saw all three of them at once.
How hard would it have been to make the kids have feature sliders like the advlts? Different colored hair at least? It's like the design team copy and pasted the same child NPC every time they needed one. It's a shame, because otherwise all the children have unique personalities, but the Children of the Corn syndrome IS harmful to immersion and smacks of laziness.
This isn't Bethesda's first go round with children either! The kids in Fallout 3 all looked different. Why the clone syndrome in Skyrim?
It isn't helped by the fact that children only seem to have one outfit. Or two, if you count the fact that the female children wear dresses. Good thing too, otherwise I wouldn't have been able to tell them from the boys.
It all just seems jarring, especially in comparison to how carefully crafted and detailed the rest of the game is . . . .