Williams, who was capped last year at the same age group, scored a try and kicked four conversions and a penalty to cap a fine personal performance whilst centre Gareth Owen, prop Bradley Davies and wing Dylan Morgan made sure of a bonus point.
Team manager Wyn Thomas said; "Obviously the conditions were a bit tricky in terms of handling the ball and there was an element of nerves due to it being the first game of the season.
"However, once the side settled, they did well and it was a boost for the side to grab a bonus point in the final minute."
Tries from Luke Hamilton, Luke Williams (2), Aled Davies, Curtis Morris and Tomos Ellis Williams put the Scarlets under 18s on course for a consummate victory.
James Russell sealed victory with a penalty and four conversions whilst there were yellow cards for Ayden Baines of the Scarlets and Gogledd Cymru's Jake Simms.