Children get thirstier more easily during hot summer days.

thirstier 是形容词 thirsty 的比较级形式。thirsty 由名词 thirst（口渴）加上形容词后缀 -y 构成，意思是"口渴的"。thirstier 表示"更口渴的"，用于比较两种情况下口渴的程度。