hmm. well, of course any vegetables. :)
to get kids to eat cauliflower, just steam (or you could boil it) a bunch of it, and then mash it and fix it up just like mashed potatoes! kids don’t know the difference.

SOUP. kids love soup, and you can pack so much into a soup. and they are really easy to make.
QUINOA! it’s an incredible grain and you can fix it up soooo many different ways.

you can buy boca chicken nuggets. they taste just like chicken, but are made from soy.

there are endless options.

or you can always give kids vegetarian burgers :)
another thing i can think of is mixing up in a boul different veggies, with different and atractive colors or something like that, it also depends on the age of the kids :)