In reply to Oo: I'd say it depends on the rock type. On rough rocks (granite, gritstone) pretty much any footwear will be fine, however on some limestone you'll struggle badly in trainers.
Decent (i.e. leather) walking boots with a traditional Vibram sole are generally an excellent option. They will give good edging performance and will also smear well. Unfortunately, the vast majority of cheaper fabric walking boots of the type Scouts are likely to own these days won't be much better than many trainers.
As mentioned, 'Primark' or other cheap plimsoles are a decent option, but I'd have a check in a store before hand and give a fairly specific
recommendation if you find that they have something suitable.
The only other thing worth emphasising is for the Scouts not to wear anything particularly expensive if you are climbing on rougher rocks. Granite or grit cracks can trash trainers or 'fashionable' approach shoes in very short order.