Children who accurately guess how much was in their piggy bank(猪形储钱罐) get a prize, and free candies are handed out when the coins are exchanged for banknotes.
准确猜出自己猪形储钱罐里有多少钱的孩子可以获得奖品;把硬币兑换成纸币时,还会免费发放糖果。
这是限制性定语从句,"who"在从句中作主语,修饰并限定先行词"Children";只有准确猜出金额的孩子才属于该范围。