What Itachi did was horrific, there's no denying that. You have no manga facts to back up the claim that he murdered more children than anyone else though. It was never stated how many children were in the Uchiha part of the village. We know Oro killed at least 59 children before he left the leaf village as Yamato was the 60th one and the only survivor. There are no canon facts to say there were more than that but we do know that he still was conducting experiments and that Karin was Sasuke's and Naruto's age, so he was still experimenting on children. We just don't know how many more died.
Unless you can state that you know exactly how many children were in the Uchiha part of the village, you cannot know for sure that he murdered more children than anyone else. I'm not an Itachi fan so this statement is not biased. Until you can prove how many children there were in the Uchiha clan this is not manga fact.