...Perhaps George Kirk Sr. pulled a few strings to have his wife whisked to Earth for childbirth on state expense.
Regarding Tarsus IV, we not only have to wonder why young Jimmy faced Kodos alone, but why only nine people out of 4,000 faced him at all, and why two of them (Jim and Kevin) and perhaps more (Thomas Leighton wasn't all that old-looking) were mere kids. One line of reasoning would be that Kodos wasn't one of the colonists at all, but a person who came to Tarsus IV on the same ship as Jim and Kevin and Tom - which is why these three knew his shipboard identity, whereas the 4,000 survivors only knew the Kodos identity he assumed when taking power, an identity worthless to investigators. (No doubt, the man who became Kodos would have been traveling under false identity on the ship already, but at least that identity would be one step closer to a proper trace.)
We could speculate Winona traveled from colony to colony as part of her profession, taking her children with her since George was unavailable and her brother Frank couldn't be trusted - and perished in the hands of Kodos, which is why she doesn't count as a witness despite having enjoyed the same view of the pre-Kodos Kodos as her son did. (Although why George Jr. wasn't with them, or was but didn't become a witness, remains unexplained then.)
The episode describes Kirk, Riley and Leighton as eye witnesses not as members of the resistance. They are said to be survivors of the massacre. I would infer from that they were in the group rounded up for execution and only lived because the relief ships had arrived and Kodos apparently died. Kodos seemes to be in power when the famine started and is described as taking "full power". What the other 4,000 people, the "superior" ones. were doing at this time isn't really said. Surely they knew what Kodos looked like. Kirk found a picture of him easily enough and had access to a recording of his voice. Possibly, only the unfortunate 4,000 actually heard and saw Kodos give the orders.