Melanie,
This is an example close to my heart! Early childhood education actually has had a few rigorous RCTs, which suggest enduring benefits. For example, Perry Preschool, and the Abecedarian project both randomized children to enriched early education programs. The limitation of those studies is that samples were small and so results have a fair bit of noise. There are also major questions about whether benefits are highly dependent on the implementation or on the particular population. It is difficult to replicate careful, well done projects that were labors of love in a small community when you scale something to large, diverse settings.
This seems like an area where there might be good quasi-experimental evidence, which I think has been used for Head Start evaluations, for example.
Maria