Soy protein is not an effective alternative. It is high in allergens
(some 28 different proteins present in soy have been found to bind to IgE antibodies). It's also worth noting that the more soy protein you eat, the more likely you are to develop allergies to it -- and the more severe those allergies are likely to become. Soy also blocks the absorption of important minerals such as calcium unless the phytates
have been removed, and soy contains high levels of phytoestrogens, which although beneficial in moderate amounts, can be counter-productive in large amounts -- particularly for children.
In addition, although its biological value is not bad at 70-80, it's net protein utilization at 61 is quite low. In fact, unless it has been fermented, soy protein contains potent enzyme inhibitors
that block the action of trypsin and other enzymes needed for protein digestion. This can create significant amounts of gas, in addition to promoting pathological conditions of the pancreas, including cancer. As a side note, soy protein was once considered a waste product of the soy oil industry and used almost exclusively as cattle feed.