MacIntyre, Baker, Clément and Conrod created a WTC questionairre. It is widely cited and adapted.
I do not know whether it is suitable for your age group, but maybe you can adapt it, or maybe someone else has adapted it to learners of your participants' age.
SEe: MacIntyre, P. D., Baker, S. C., Cle´ment, R., & Conrod, S. (2001). Willingness to communicate, social support, and
language-learning orientations of immersion students. Studies on Second Language Acquisition, 23, 369-388