You definitely want a youth-specific balaclava for your child. Adult sizes—even stretchy one-size-fits-all models—are proportioned for adult head shapes and neck lengths, so they'll leave gaps around the face and neck that let cold air and snow in. Those gaps aren't just uncomfortable; they can be a safety concern in extreme cold.
Youth balaclavas are scaled down in every dimension, with a smaller face opening that sits properly against a child's features and a neck length that actually covers their neck without bunching or riding up. They're designed for kids roughly ages 6–12.
It's tempting to size up and let them grow into it, but a loose balaclava won't seal out the elements and can interfere with their goggles. A proper youth fit keeps them warmer, safer, and more comfortable on the mountain.