The problem with the child...and the problem with vegan and strict vegetarian diets is the lack of B-12.
Without supplements, children especially can develop severe neurological problems.
I saw also a link to higher rates of Alzheimers amongst vegetarians...again thought to be due to B-12 deficiency.
Still...a balanced vegetarian diet with some supplements >will< reduce risk factors for heart disease...but so will a well balanced diet and exersize.