Basically you'd need to know how common crime against children traveling unsupervised on buses in a similar scenario.
The closest example presented is kids in New York riding the subway, so presenting stats on that would be useful.
I'd also like more details on the Japanese example, it's honestly not uncommon to hear news reports of kids being abducted in Japan. I'd personally be interested in the stats, but as a comparison point it doesn't make sense unless you can normalize for the overall lower crime rate that Japan has.
If anyone has good data against "Helicopter parenting", I'd be curious to see it.