Youth-specific ski socks are absolutely worth it for kids. Adult socks—even sized down—are proportioned for grown-up feet and calves, which means excess fabric will bunch up inside a child's ski boot. That bunching creates pressure points, cold spots, and blisters that can cut a ski day short and make kids miserable.
Youth socks are scaled down in every dimension: foot length, heel pocket width, arch placement, and calf circumference. This means the cushioning zones actually land where they should, and there's no extra material to wrinkle or fold inside the boot. A proper-fitting sock also helps the junior ski boot perform as designed, giving your child better control and comfort.
If you're tempted to size up so they "grow into them," resist—just like with boots, a too-big sock causes the same bunching problems. Buy the right size for now and replace as they grow.