Maybe the boys themselves thought they, one or the other of them, would somehow be implicated or blamed, hence their identifying Bix and insisting it was he. It's not so far-fetched -- two working class teenage boys who were laboring in the street at necessary jobs to support their families instead of still being in high school at their age would scarcely have stood a chance at all at such an accusation; certainly they did not have socially influential or moneyed families to "get them off the hook."
Simply being poor or working class was a crime 90-100 years ago; easier to blame and arrest the poor for being drunk, disorderly, at the wrong place at the wrong time, the most likely person in the police and then a jury's eyes to steal something from a store or harass a woman or child in the street, or even to speak insultingly, really just "disrespectfully" to a rich and socially prominent person, who had the power at the time to order their immediate arrest. I've got plenty of old books from those times who recount just such incidents. (Things aren't all that different today. Look at all the people still railroaded into jail or forced to remain there when they can't pay the bonds or fines of things they are only accused of but not found guilty yet in court. Or how much harsher and punitive sentences are to proven culprits of even minor drug offenses, who aren't wealthy or celebrities. The rich go to rehab -- even if they plow their car into crowds of people while they are smashed. The poor get caught with a bag of pot and go to prison for the next 40 years. And even more cruelly punished than poor men are poor women. And all minorities.)
Pointing at "some rich kid" privileged enough to remain in high school -- and to them an upper middle class boy, and one going to public instead of private or boarding school at the time, was STILL a rich kid -- might have eased more than one emotional and social burden off of the boys' shoulders. Whether or not they were acquainted with Bix, they understandably could have resented his social class. Had Bix been just around the area at the time, it would be enough for them to say, "It was him, sir, he was the one who took the little kid into the garage," one or the other nodding assent, "Yeah, it was him." Whether they were sure of it or not.
But I am more inclined to believe that, less out of vengeance, they might have been scared or at least uneasy -- perhaps thinking or discussing, what if that little girl said it was one of them? She couldn't see well. That Bick guy can't get into too much trouble; his folks got lots of dough.
Whatever the situation and circumstances, the hard fact is that anyone of pedophile tendencies "doesn't do it just once, and never attempts it again." Being caught is not enough to scare someone of perverted proclivities, and never again was Bix suspected, accused, or arrested for molesting or attempting to molest a child. His friends and colleagues, however protective of his memory, readily and sadly admitted his other faults and shortcomings in his lifetime and in interviews decades after his death, and for all the other socially unsavory aspects brought up about his alcoholism, emotional disengagements and messy lifestyle, this particular calumny was never even alluded to.