The choice of what is on children's menus is culturally determined - it is whatever is generally considered to be liked by most children in a particular country.
So, for example, in the UK children's menus tend to be full of fish fingers, pizza, sausages and so on, because most of them will have eaten those things at home - not all the time, but they are familiar. In France you find more ham, omlettes, steak etc, and in Japan, it is often tuna or cod roe sushi, prawn tempura, frankfurter sausages and potato salad. Not a conspiracy, just restaurants seeing a demand and catering to it.
Yes, TankFly, I am veggie and the DCs are veggie plus fish, so in a lot of restaurants the veggie options not on the children's menu don't appeal to them. Though if we go somewhere like Yo Sushi or Wagamama they are very happy to take their pick of the adult offerings.