Taking pictures of kids on bikes isn’t that hard, really. Just shrewdly choose a well placed bind in a strategic spot, take position, and lurk (erm, I mean just patiently wait) for the right moment.
Not hard if you’re a patient person, and don’t mind risking getting picked up as a perv. Me, I’m neither one. I prefer to just keep alert and hope to get lucky now and then, like I did in Seville’s Plaza Espana.