https://www.theguardian.com/environment/2025/oct/29/soil-sandpit-children-dirty-biodiversity-finnish-nurseries-research-microbes-bacteria-aoe

Finland study focused on "microbiome" or microbe-exposure benefits to being outside, playing in dirt.

I'd suspect that getting slightly more sunlight could help too. Anyway, lack of outdoors was problem for lots of people in covid lockdown. Not me. I got into woods & fields, unmasked & often. But some people stayed at home, depressed & unhealthy.