It's not like they breed the cows and kill them for the leather, surely leather is the by-product after us non veggys eat the meat - they are therefor recycling the animal skin that would otherwise end up in landfill (or pies and sausages).
As noble as ther ideals are, plastic or canvas shoes aren't as good for your feet, or as robust as leather - canvas being the better option, but they dont usually give the appropriate support and wont keep out the rain.
I also hate welly boots for the lack of support they give to growing feet - fine for the odd trip out in the rain, but you sometimes se kids wearing them for the best part of the day.
Their best option if they really cant face the leather, is to get a really good pair of breathable synthetic trainers, might be difficult to get them without leather trim though.