Two goals from Pablo Fornals and a deflected effort from another teenage debutant, Divin Mubama, ensured the Hammers - already through to the last 16 of the Europa Conference League - became the first team in the competition to win all six of their group-stage matches.
Manager David Moyes was able to give 14 senior players, including Declan Rice, Jarrod Bowen, Kurt Zouma and Michail Antonio, the night off.
Yet it was still a mature performance from a line-up including first appearances for midfielder Scarles and homegrown striker Mubama, 18, as well as rookie substitutes Freddie Potts, Keenan Appiah-Forson, Kamarai Swyer and Kaelan Casey.
Scarles, who became West Ham's sixth-youngest player, told BT Sport: "It was just a surreal experience, just to make my debut and be with these boys.
"Yesterday and today has been an amazing experience and something you dream of.
"It was Wednesday morning when we'd done the first training session and (Moyes) sent a few of my lot to do finishing practice. Then we had the 11 starting in positional practice and I was in that, and my heart dropped!
"I was like 'oh my god, I'm starting'. It was mental."