If they try withdrawing an occupied tray from the stack, I'd expect them to break the fibres and cause all manner of alarms to go off. If they pull an unoccupied tray from the stack, it's just a damaged tray that is probably easy to replace.
From my understanding of how it all goes together, it's quite a disassembly job to get inside a fibre manifold, especially if it's in an underground chamber. You've got to lift the chamber lid (which isn't easy without the right tools), lift the framework on which the manifold is mounted, unscrew the manifold cover and open the strap that retains the trays. The chamber is probably full of mud and possibly full of water - I just don't see the motivation to interfere.
Few kids will see any sort of devilment in messing with the manifolds - there's far more immediate ways to get your kicks whilst causing criminal damage. There's no money to be made, as there's no metallic cable to steal. Most won't understand how the network goes together anyway. Few will want to risk the ire of their friends when they are unable to play the latest Xbox / PlayStation game online because some twit broke the broadband network.
There's plenty of Virgin Media nodes around here with loose and unlocked covers. I've never seen the kids interfering with them, even though it would be easy to lift off the lid, unscrew all the coaxial cables from the tap plates, pull all the twisted pairs from the Krone blocks and pull off all the labels so it's hard to put it all back together again. The kids probably care far too much about being able to watch television (this area has poor sight lines to the south, meaning satellite often won't work) and cable modems (ntl:, as it was then, got here long before any DSL) to want to interfere.