Brain requires a lot of energy to work, so better nutrition will certainly not hurt. However, I believe that if there is some improvement to intelligence due to better nutrition, its effect is negligible compared to improvements due to education, better teaching techniques and near-universal literacy.

I hope other non-educational improvements in child rearing have also had a significant effect. In particular, the widespread acceptance of the idea it's not okay to strike children has hopefully greatly reduced the number of children trying to learn with varying degrees of PTSD.

Instead, parents feel bad about telling their children what to do, and without rules and structure anxiety and depression seem pretty high.