It might be okay for very young children. Even I don't think that five-year-olds should be walking to school alone -- preferably their parents should walk with them, but if that's not an option due to work schedules then a walking bus isn't a terrible idea.

At eight or so, though, sensible children start to have their first rebellious streak and realise how stupid the idea of a "walking bus" is. I figure that when they're smart enough to figure that out, they're old enough to walk alone.