No big shock. Coal is loaded with heavy metals, its burning indoors limits oxygen availability (both by reducing oxygen levels and by blocking binding sites with carbon monoxide), etc.
True enough, we’re taller than previous generations for a whole lot of reasons. Cleaner air in the home is likely one of those reasons.
When I was a teenager we took the old coal furnace that sat in my great grandmothers basement and made it into a wood furnace at our house.
Coal varies greatly in composition from area to area. Most coals I have seen are hardly "loaded" with heavy metals. You would have to breath smoke constantly to get enough heavy metals to make a difference similar to the lab rats eating a steady diet of saccharine just to make them get cancer.
Second, you seem to assume the coal stove leaks flue gas into the building. It is possible for a flue to be properly constructed to carry flue gas outdoors. In addition, the stove burning inside a building quite possibly could cause better air quality indoors by bringing in fresh air due to the draft.
As others pointed out it is possible the children are shorter due to poor diet since people who heat with coal are more likely to be poor. I'd also say it is possible this is junk science. I don't recall hearing about short blacksmiths in the old days.
Unless the coal is burned in the open inside the home and not inside a stove vented outside, how is it reducing O2 or has enough CO to block but not asphyxiate those using it for heat?