Well, if you don't trust the North Korean sources, you probably shouldn't trust the South Korean sources either as most information I've found was published in the late 90's and early 2000's during the height of the liberal administrations of Kim Dae-jung and Roh Moo-hyun. They were focused on the Sunshine policy and keeping North Korea happy. Considering that lots of the information came from the government sponsored Truth and Reconciliation committee or was broadcast on government controlled media, I'd say that it's all suspect. That said...

Did it happen? Probably. Did it happen as we know it? Probably not.

The claims:

On October 21, they drove five military trucks full of inhabitants, whom they drowned by unloading the trucks in the artificial lake of Sowon. A total of 1,640 people were drowned from the end of October to the end of November in Sowon and Pogu lakes.

On December 7, US soldiers under the command of Harrison kidnapped the children […] imprisoned in a former powder warehouse in […] Wonam, Sincheon county […] and killed them by giving them gasoline instead of the water they were asking for. The surviving children and the mothers were covered with gasoline, and they threw fire and grenades at them. More than 400 women and 102 children passed away.

North Korea makes these claims in their museum in Sinchon, the Sinchon Museum of American War Atrocities, but there is no evidence to support their claims that US forces did it.

The reality:

Both Koreas engaged in systematic targeted killing of political opponents and dissidents. South Korea murdered communists and left-leaning prisoners as the war began in an attempt to prevent them from falling into the hands of North Korea. After the UN/US forces pushed North Korea back, the South Koreans killed those who collaborated with the North and even people simply suspected or accused of collaborating with the North. The Goyang Cave massacre, the Namyangju massacre, the Jeju uprisings, the killing at No Gun Ri (which was done using US forces) - these are all prime examples of ROK forces or right-wing groups killing suspected left wingers. This is the same situation that caused the huge evacuation of Hungnam in December 1950 - people in the North who had collaborated with the US/UN forces had to flee out of fear of retribution.

Two authors, Hwang Sok-yong and Bruce Cumings have both been to the Sinchon museum and both agree that a massacre did take place but they doubt American participation. Hwang, in his book Sonnim (The Guest), claims that it was Korean communists that committed the massacre against other, rival communists. Bruce Cumings discussed the massacre in his book, War and Television, and came to a similar conclusion. With US forces controlling the area, Koreans settled their own scores with fellow Koreans.