One, even non-diesel buses are smelly and bad for the environment. Even a 100% electric bus would be bad for the environment, as right now most of the world’s electricity is produced via polluting means, so increasing the amount of electricity required by each city would increase the number of generating stations thus increasing pollution levels overall. Diesel buses are still better than each parent driving their child in by themselves, not only for the environment but simply due to the number of car accidents that happen each day.
Two, who exactly is going to pay for the new fleet of buses? No one wants to, thus it’s not done. As well, “If it ain’t broke, don’t fix it” is generally the mindset many companies take. But this can be combined with making buses more eco-friendly. For instance, in my city, whenever a public bus finally dies, they replace it with an eco-friendly bus which I believe contains hybrid components.
Three, pretty much everything is more toxic to children due to their small body size. I would be interested in seeing some statistics regarding diesel versus gasoline fumes in relation to a child’s health, as well as overall child mortality statistics, as right now I cannot agree that diesel fumes pose a serious risk for children in relation to other risky activities they take part in. If your main concern here is the safety of children, there are many other ways this can be accomplished.