Scally Quandary

take photos of them and inform the local police about it, people nowadays have camera that have a decent zoom (not digital zoom).

use those photos in your 'letter', pointing out that these children *will *nick part of your bikes or possible the bike itself, etc.
**
Or for best result, take photos, put them in a poster, and post it near bike lock, the kids will notice that their faces is plastered on a poster and realise someone is watching them**.

probably won't deterred them, but worth trying.

+1 do it!