Chain walking off the sprocket usually means the chain is loose or the sprocket is worn, not the willow's fault. A wet willow will make it worse because of drag, but a properly tensioned chain should not pop off mid cut. Check the drive sprocket teeth for hooking, once they wear down the chain climbs right off. Also check your bar groove for burrs from that pond landing, that bar might be done for. Tension it so you can just barely pull the chain off the bar by hand at the middle. Cut wet wood all you want, just retighten more often because wet chips pack the sprocket nose.

A 250 at 12 is still a 250 at 12, and that bike will do 60 plus whether the kid respects it or not. My buddy's son was on a 125 at 13, hit a rut he'd ridden a hundred times, and broke his collarbone and two ribs before he could even think about the clutch. Being good on a 90 for five years does not mean your body and brain are ready for double the power and double the weight when something goes wrong.