I believe not seeing faces has real long term effects on people. Less human interaction is detrimental both for the individual's psyche and collectively as a group. I think people are less likely to recognize the humanity in others when they can't see their faces. We're often too quick to dismiss other people as simply being 'other' even without masks. Faces, expressions, smiles connect us. I think people are often disarmed by a smile, even (especially?) when they come from strangers. We communicate better by seeing each others lips move to form words and simply because it's easier to hear someone who is unmasked. I think for many people those small connections help them feel less alone. Again, I know these are small things individually, but they add up for society and for the loneliest individuals over the course of nearly two years. That's why I say the costs are insidious but no less real.
For kids, all of the above of course. Add to that the problems masks cause in the classroom...I don't need to outline those, just ask the nearest teacher.
Hard to quantify these things, but I believe they are very real and very costly in the long run.