A Question of Courage by Megan Magill

Jess hadn't been looking for another adventure, her life was disordered enough. Still, here she was, embroiled in who knew what.

When Nick had first asked her to come and work at the children's activity camp with him she'd baulked. She had no experience of children and had never tried to teach archery to anyone else. He had pleaded with her though, citing fears of sinister goings on. A combination of his little brother act and her need for gainful employment won her over. At first, she hadn't put much stock in his concerns, assuming they were fears arising from his overactive imagination. Soon after arriving at camp though, Jess is forced to change her mind.