noun
snowball rolling game
A children's winter game where players roll a snowball around to make it grow larger, often competing for size.
雪が積もったので、子どもたちは雪転がしをして遊んだ。
Since the snow had piled up, the children played the snowball rolling game.
雪だるま is a snowman, the typical result of stacking snowballs, while 雪転がし refers specifically to the game of rolling a snowball to make it bigger.
Compound of 雪 (yuki, 'snow') and 転がし (korogashi, 'rolling'), from the verb 転がす (korogasu, 'to roll').