There's a lot of Japanese quirks I still don't get and giving peppers to kids is one of them. I guess it's like Americans and celery but that's dumb too.
@HiraOmnibus Because green bell peppers are overwhelmingly reported as a thing children don't like and are the most resistant to eating in Japan. If you mean mechanically why that's because they are unripe versions of their more colorful counterparts; they have less water, less sugars, etc and thus have a more pronounced bitterness to them. Keep in mind young children have more sensitive palates than adults so they perceive that bitterness far sharper than we do once we're older; combo that with our basic instinctual understanding of what bitterness means, poisonous.
@HiraOmnibus Parents routinely give kids foods they don't like, because the only way to get them to eat stuff is to make them keep trying it over time. Some stuff they'll never like, but many things they'll eventually decide they like it after trying it enough times. This is especially true of healthy foods like vegetables. If you let kids only eat what they like, they'll have an absolutely abysmal diet nutrition-wise.