Why would any child need total parenteral nutrition?

Intestinal problem. Tpn is nutrition given via an iv. It is quite tricky, and only used in cases where the intestinal tract is very, very impaired.
GI tract problems. If a person can not absorb nutrition from their intestinal tract, they get it into their veins as total parenteral nutrition. Sometimes its given while the intestinal tract is recovering or for additional calories in children who cant get all the calories they need through their intestinal tract due to their disease. There are conditions where children are totally dependent on TPN.