Without wishing to sound like a grumpy old bloke, one of the most depressing sights around here and, it seems nearly everywhere else, is the proliferation of mindless tagging on every surface, spilling out from the tolerated sites under the SBC onto the beautiful concrete superstructure and causing visual noise everywhere you look.
Rather than encouraging this anti-social "art-form"/crime, why not encourage your charges to take part in a less anti-social cultural activity? There are plenty of art supplies shops around. Perhaps they could try painting on paper or canvas?
I know you are looking for a space where it would be permitted (and thank you for that), but providing such facilities legitimises and encourages the activity.
ps I was gobsmacked the other week to see a middle-aged, middle-class looking dad (baggy shorts of a non-descript colour - you know what I mean) encouraging a child (presumably his) of about 13 to spray paint a bridge in Crouch End. I'm all for parents being involved in their children's activities but I bet he'd be up in arms if someone came and painted his house or car.
Rambling Phil - the thing about grumpy old blokes (sic) is that they are usually grumpy with good reason. I have no idea whether you're old, but your grumpiness in this instance seems entirely justified. My intended but implicit point was that organized graffiti legitimates something that is meant to be subversive and is also antisocial.
And I didn't spend twenty years being mildly concerned about the prospect of mutually assured nuclear destruction while going about my daily business so today's kids could listen to tinny sh**e music on the top deck of buses and spray paint anything that isn't brightly coloured and being complicit in a celebrity-infexted, consumerist late-capitalism.
Many moons ago i took my 8 yr.old stepgrand-son who had lived in S.A all his life under the subway in new kent road, he looked at all the graffiti which consistants mostly of 'tags' as they are now known, and asked who drew on the walls, i replied probably school children..and with the innocence of a child asked ' do they not have any paper then?'.
I have seen some graffiti that is dramatic and has been carried out by someone who had skill, but not enough to justify defacing public and private property.