Hardshell (ABS) construction is usually the smarter pick for kids. These helmets have a tough outer shell that's glued to a separate foam liner, making them much more resistant to the everyday abuse kids dish out—getting tossed in gear bags, dropped on the parking lot, or knocked around the lodge. In-mold helmets are lighter but their thinner shells dent far more easily from those casual bumps.
The weight difference is real but modest, and most kids won't notice it. What they will benefit from is a helmet that stays intact season after season. Hardshell helmets also tend to cost less, which helps when you're replacing them as your child grows.
One important note: hardshell doesn't mean better impact protection. Both construction types meet the same safety standards for serious crashes. The difference is durability against daily wear, not how well they protect your head in a real accident.