I've been vegetarian since I was 16, and my DCs have been predominantly vegetarian (they eat some fish, so technically pescetarian) since birth, and are very big, healthy children. We eat lots of fruit and veg, and protein comes from beans, lentils, tofu, eggs, cheese, milk, nuts and the occasional meat-substitute thing like vegetarian sausages.
It's really not hard to get enough protein in a vegetarian diet, and the combined-proteins thing is a bit outdated - these days, most dieticians seem to agree that as long as you get a range of proteins in your diet over a period of a day or two, there is no need to worry about combining foods to get complete proteins at every meal.
A lot of people just seem to take the lazy approach and substitute quorn/soya versions of meat products for most meals, but I don't think it's healthy to eat that much heavily-processed food, and making things out of fresh veg and pulses is much nicer. Simple things for children could be pasta with a bolognese-style sauce made with lentils instead of mince, and cheese sprinkled on top, or veggie burgers/sausages made with beans & veg held together with egg and breadcrumbs.
There are lots of vegetarian recipe books for children, but I find that the internet is the best way to find recipes these days.