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.