Kids are going to be effected by the current era in huge ways, just like every generation is effected by the the world events surrounding them. The disruption of schooling, domestic political conflict, imminence of mortality, the fact that other people are viewed as possible vectors of disease, the fact that the very idea of contagious disease is more prevalent that at any time in decades - these will all shape a generation, in the same way that war or terrorism has shaped previous generations. I have no idea exactly HOW these things will shape the generation, but it will.
In the grand scheme of things, the effect of masks on children pales in comparison to these broader forces. Considering the broader effects of this pandemic, focusing on how much masks effect childhood development seems a bit limited.