Protein isn’t so much the problem as, potentially, some vitamins and minerals, especially a couple of the B vitamins, and maybe calcium. Many millions of children have grown to adulthood quite healthy on a “standard vegetarian” diet (one that includes animal products but no “dead animals”), and many millions more without signficant amounts of “wild salmon (or other seafood-derived) Omega (-3 fatty acids)”, too.
The bigger problem, as I see it, is that many people who don’t belong to communities that practice “traditional” versions of a vegetarian diet (or don’t otherwise uniformly follow their practices) don’t inform themselves well enough/aren’t careful enough to ensure that their diets do provide all necessary nutrients in appropriate amounts at all stages of growth. (And that’s even more true of a vegan diet, which as far as I know, is not something “native” to any human culture except in dire, subsistence-level conditions). But if governments determining what people’s children should and should not eat (in the absence of emergency health situations) were considered an acceptable norm by most “modern, enlightened” governments, there would presumably be far fewer obese children in the world, not to mention of course no starving ones…
