I doubt whether a four year old would ask why all the people ‘rampaging’ in the photos are men to start with. I think we can safely assume that the older child asked. But even if it was the younger one, a four year old gets then/now.
All we know from the podcast is that his daughter (whichever one it was) went down a series of pictures of spitfire pilots and went 'boy','boy','boy','boy' - Dan Snow's response was to show her some pictures of women spitfire pilots and said, there were women who flew spitfires too. To my mind there is no lying anywhere in that scenario - Snow was correct to point out that there were also some women who flew spitfires in response to his daughter's comment that all the pilots she saw in the pictures were men.
Clearly Snow recognises that there are elements of the story which he has missed out in his response, but I don't think that means he was lying. And when he talks about it he doesn't say 'I lied', but the more nuanced that he 'sort of lied', which I think is a recognition that he was guilty of omission rather than overtly saying something that wasn't true.