Sure it is.Just ask anyone who has or knows someone with PKU.We DO NOT need meat(although I love it dearly).Two of my children are phynelketonuric.That means their bodies cannot break down the amino acids in protein.Meat is very high in protein(well except for bacon fat),so they cannot eat any meat.Nor can they eat or drink any dairy product.They even have to watch some of the vegetables because so many contain lots of protein.Potatos and english peas come to mind. (My daughter,11 years old can only have 6 grams of protein per day.Look at some of the nutritional values of some foods when you get a chance.One slice of bread is around 3 grams)
That said,they are extremely healthy.My 11 year old daughter is about 100lbs(she's tall) and my 1.5 year old son is around 25 lbs.They are measured throughtout the year to make sure they are growing as they should be.They're usually growing above the normal percentile.Something else is wrong with this picture.These kids are way under fed or have some type of disorder.It's not the lack of meat or dairy products.
That's a very specific genetic disorder, in which even tradiional staples of vegan diets, like beans and peas, will result in mental retardation, seizures, etc. I don't doubt that affected individuals are healthier if they don't eat any animal products, than if they did (though probably not as healthy as if they didn't have the disorder, and could eat normally). I'd be really surprised if your daughter isn't getting regular dietary supplements of some kind, of synthetic and/or animal origin.