Beef is a particularly inefficient protein source.
It's not the product, is the producer. The perverse incentives in our meat and farming industries drive production of a vasty inferior product at an unsustainable price.
The last food-animal-related health problems I heard of were multiple different bird and pig flu varieties, the last major problem with cattle was BSE/Jakob-Creutzfeld disaster years ago. So I think the cost of perfectly fine, but "preventively killed" animals should be weighed in, too.
TL;DR: It is extremely difficult to sustain the needs of a child with a pure vegetarian/vegan diet.
Tripe. I think people forget that even with the naive diet pyramids a lot of us grew up with, eggs (rightly) were counted as meats. It's been well understood for a long time how to function without eating meat, as in, the flesh of animals, if you have eggs available as part of your diet.
Pure veganism requires some actual thought, yes.
So really, meat is wholly replaceable in the human diet, though doing so in children may be difficult.