It causes water intoxication which can lead to seizures and coma. That then can affect the brain @nichole_lynn. It fills them up too much without satisfying the hunger. Poisoning the baby.
the baby water or nursery water is for making bottles of formula, its purified water basically. no harsh chemicals, like water that comes out of the sink @starr408 and at a young age water can hurt a babies bran apparently. (I just learned this)