That age group (and all others) should be eating real/whole fruit or having the juice fresh (I.e. just juiced). They would be better served getting this advice than creating more anxiety about protein intake.
> Commercial orange juice with a long shelf life is made by pasteurizing the juice and removing the oxygen from it. This removes much of the taste, necessitating the later addition of a flavor pack, generally made from orange products.
> Commercial squeezed orange juice is pasteurized and filtered before being evaporated under vacuum and heat. After removal of most of the water, this concentrate, about 65% sugar by weight, is then stored at about 10 °F (−12 °C). Essences, Vitamin C, and oils extracted during the vacuum concentration process may be added back to restore flavor and nutrition.
So essentially there are components that vaporize during processing. The make sure to condense the same components and add them back in so that the orange juice contains all the components of fresh orange juice.