Would adding an extra child model or two set you back that much? Jarring, immersion breaking, plain LAZY. All of the kids are recycled everywhere. For a studio that harps on attention to detail this is a lazy oversight.
This complaint is ridiculous. What were you expecting, to have all the races represented in a land dominated by the Nords?
I don't know if they pegged the space limit, but you can only stuff so much data on a retail disk. Having more variety in the child's model only eats up said disk and leaves less room for more meaningful content.
So maybe it was an oversight, maybe it wasn't. Just be glad they're represented because unlike the other ES titles, they were non-existent. Besides, there's dozens of other activities to do besides taking note of the single child model in the game. Just load up and play.