Re: How many kids did Itachi kill during the massacre?
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Sasuke couldn't have been obky child in the Uchiha clan.
No, I doubt Sasuke was the only child. I imagine there was probably quite a few - they probably didn't marry outside a lot (or bring outsiders in), like I think the Senju probably did. With their bloodline and clan identity, I imagine them to be like the Hyuga - probably inbred to some degree...
With their history, clan identity etc I think it was important for them to spawn plenty children to keep the Clan and bloodline strong. Though I suppose the Third War (and the two previous) had a serious impact on their numbers, all the more so when you consider their capabilities/specialisations and how they'd likely their kids were the first to be turfed out the village and onto the frontlines.
And like the Uzumaki, they were feared - that fear would make them a target so...
Whatever. There had to be enough to keep the Clan strong - to keep their numbers plentiful to fufil their duty as the strongest Clan in Konoha, to staff the Police Force etc.
They had to have decent numbers if they felt it was even remotely possible this coup could work - they'd need a strong generation to follow =
lots of kids for Itachi to kill.
Also why I don't think Sasuke was the youngest. He was what, 7 years old? The Clan had no children under the age of 7?
... no. effing. way.
That makes no sense, if you want to be remotely realistic about sustaining a large clan like the Uchiha.
One other thing - that image of Itachi dragging the kid out from his hiding spot? Highly inaccurate portrayal of events. No kid would have heard him coming. No kid would have moved fast enough to put a leg out his bed, nevermind make it across the room and into a closet.
Plus Itachi wouldn't have allowed that vase to break... or the side table to fall, or the painting (?) to slid down the wall...

Nuh-uh.
Ain't hapening.
Itachi would have snapped his neck like a twig before he even opened his eyes - just saying =D