These games are suitable for children who are developmentally behind their peers.

developmentally 是形容词 developmental 的副词形式，由名词 development（发展）加上形容词后缀 -al 构成形容词，再加上副词后缀 -ly 构成副词。developmentally 表示"在发展方面"，指儿童在身体、智力或社交等方面的成长发育情况。