When you view all this from the angle of "he's an AGP, but for kids instead of women" it explains a lot of the weirdness. AGPs fetishize the concept of womanhood while also not understanding it outside of its most superficial trappings (dresses, breasts, makeup, etc.). David's understanding of being a child is also focused on its most superficial aspects: Having to go to school, having parents who look after them (hence the "This account is monitored by my mum" line in the Instagram bios), having to wear a uniform.
These are things he focuses on because he perceives the fact children have to do these things as being unpleasant for them, the same way that AGPs are motivated by the underlying misogyny that being a woman is a "lesser" state than being a man and so to become one is humiliating. He doesn't care about how children talk or act because that's not relevant to him, what's relevant is what society does to them because they are children.
Kids do not like wearing school uniforms, even though they have to. That's the source of arousal. That's why he wrote letters to newspapers defending uniform policies, he was turning himself on at the idea that his letters would influence people to keep making kids wear uniforms.
David is not the only pedophile who thinks this way. I remember there was a guy on Yahoo Answers who used to post questions along the lines of "I made my child wear his school uniform at home as a punishment, is that okay?" and then respond with sockpuppets of himself agreeing and saying they should extend the punishment in various ways (make them wear it over the summer, for example). I know it wasn't David because I remember the writing style being different.
None of this explains the pale skin or the big red lips, though. Who the fuck knows what's going on with that.
