Children should have free time[ ](relax)at school

to relax
relax是动词
意为休息
前面的＂should have free time＂是为了让孩子们休息
是因果关系
又因为＂have free time＂可看做一个整体的行为,是个动作,不能直接接名词,所以用动词不定式to relax