I'm a proponent of the 'Crayon' theory of Hirano's shotacon origin.
As a child, Hirano discovered the Red crayon and discovered he could get lots of attention by drawing red scribbles and calling it 'blood and guts'. Of course not all that was good attention, but to the attention deprived, the kind of attention usually doesn't matter.
In his teens, Hirano discovered the black crayon. He could suddenly give the blood and guts depth and make them more realistic. In fact, if he used mostly black and just a few touches of red, it make his scribbles look really dramatic.
Black crayon gave way to black ink, of course, as Hirano began to publish, but the black crayon is still there.
Then Hellsing got popular and Hirano discovered he had to start using more colors simply due to the realities of producing for money. He went back to his crayon box for inspiration, and what did he discover?
That's right. Hirano discovered the Pink Crayon, thin, pointy, and untouched. Once he touched the hard, slender length of the Pink Crayon, it was all over for him and Schrodinger was born. Lolicard and Loli-walter emerged shortly after.