gracesmum, with three daughters, I had the same worries as you did, especially as my maternal grandmother, my mother, my sister, and two of my nieces have all been anorexic, some very seriously. Fortunately, my older daughters have not been affected. DD3 has what I believe is called Selective Eating Disorder (started from about two and a half before which she ate everything offered) but she is one of the healthiest kids in her school (least often absent) and has been on the seventyfifth centile for height and weight all her life. I'm slightly worried about her teenage years, still to come, but she knows if there is ever a health issue because of her eating, I will seek professional advice immediately. She's fine with that.
Her brain is fine too.
re people who are very sensitive to the bitterness of green veg, apparently children are more sensitive than adults to the toxins (all plants contains some toxins, to a greater or lesser extent, in the attempt not to be eaten) that cause the bitterness. Maybe that's all it is with kids refusing veg. Most do grow out of it after all.