It’s included, but only strongly suggested......nothing graphic
You see the girl from afar....on a balcony
I remember the ‘ director’ accusing Johnny of ‘ruining ‘ his perfect set up with the young girl....
....because Johnny got her to fall in love with him
It was haunting
I'm very familiar with the Woltz's expressing to Tom Hagen how much he hated Johnny for stealing his girlfriend, but I don't think that girl was the same girl as the one he abuses in the scene in the book. For one thing, Wolta makes his complaint to Hagen at the same time the beautiful blond child is in the house, although Hagen doesn't know it then.
For another thing, Johnny Fontane is a womanizer, adulterer and drunk, but he's not a pedophile. The Godfather would have cut him off (and maybe had him killed) if he had been.