In reply to Sara G:
It's probably worth bearing in mind that - depending on weather, time of day, etc - there can sometimes be quite a few non-climbing people in the general vicinity of the top of the crag. There are several footpaths up there, plus a golf course, so it can be a popular area for walking - or indeed just 'hanging out' on a warm summer evening. On my last visit there was a small group of about ten sitting around by the line of stakes at the top, enjoying the view and the odd beverage, while a couple of their number casually lobbed small rocks over the edge.
If there are no other climbers present I'd be inclined to station whichever of your charges aren't actually climbing at some vantage point within sight of the area at the top - probably up the path where it skirts the righthand end of the crag - to let you know if anybody turns up. I'd also make your rigging as tamper-proof as possible - eg rigging rope hitched directly to stakes with no tempting karabiners to fiddle with. Maybe a bit OTT - or maybe not.