The Guillaume in question appears to be up for discussion, however. Chronicle claims the Hereditary Grand Duke was there, but as Prince Guillaume is President of the Board the odds lean a little more in favor of his being present instead.
The event took place at Forum Geesseknäppchen in Luxembourg-Merl, with 23 Explorer Scouts from the Telstar Group receiving Mérite Jeunesse awards. Nearly 200 young people total were there to receive their awards for the Gold, Silver, or Bronze levels.
A little more information from Chronicle.