Well...what do you suppose it would've added to the movie, having little Santino following his daddy across those rooftops?

Not much. In fact it would've slowed down the scene tremendously, and taken much of the audience attention away from Vito where it should be and onto this little tyke nearly killing himself.

Then, after witnessing his father's crime, another scene would have to be set up having little Sonny run back & arrive at the stoop ahead of Vito without Vito ever realizing he was being trailed by his boy.

Not to mention, the audience would now have concern for little Santino, wondering how the poor kid will react to what he's just seen.

Instead, we get the absolutely flawless sequence of Vito slithering across the rooftops, never for a second losing sight of the cool, calm, pompous, unsuspecting Fanucci...shooting him, removing his wallet and the cash in it...destroying then discarding the gun and serenely rejoining his adoring wife & children and declaring his love to baby Michael on the front steps of their building.

I guess that's why FFC and Puzo decided to leave this out in the movie...

Apple