Their left winger never tracks our full-back. Do we keep attacking that side or use him as a trap?

I’d keep attacking that side, but prepare properly for the counter. If their winger never tracks your full-back, you can create 2v1s consistently. That is an advantage worth using. The trade-off is that the winger is already high when you lose it. Your nearest centre-back, 6 and opposite-side defenders need to understand that the weak-side counter is the first danger. You might allow the full-back to go more aggressively when the 6 is positioned to cover, and be more cautious when the team is already stretched. I’d also vary the attack. Sometimes use the overlap, sometimes let the full-back’s run drag a defender while the winger comes inside. Opponent weaknesses are useful only if exploiting them does not create a bigger problem for you. At U17, I would tell the players both parts of the picture: yes, attack the space he leaves; no, do not forget why he is able to stay high.