可以考虑在found前面加一个助动词 was——In Dr. Ren and Dr. Li’s studies of preschoolers 为状语，其后因主语太长而倒装：

In Dr. Ren and Dr. Li’s studies of preschoolers was found that increasing the level of participation in extracurricular activities could benefit children's cognitive and language development to some extent, but over-scheduling children could decrease the benefits of extracurricular participation or even generate negative effects on child development.

1）介词不能当主语，但是介词片语当名词使用时，是可以作为句子的主语的：

Before 7 am is a bad time to call me. 七点以前最好不要打给我。

From July to September is the peak season for tourism here. 从七月到九月是这个地方的旅游旺季。

2）原文出自 Susan Newman 博士之手，15部畅销书作者，经常给美国各大媒体撰稿，其遣词造句，可以大胆模仿。

3）如果是大陆学校考试，最好不用介词片语做主语。因为textbook English 和 authentic English 还是有区别的。

我看应该去掉。