Thanks for the tone, but David was Old Testament. The Psalm verse clearly demonstrates that a nursing infant can indeed trust. Allowing that God isn't restricted to the means that humans are, baptizing as a commandment shouldn't exclude infants. As Baptism as a 'work' it is God's work He provides the faith, He the forgiveness. Water without God's Word is water, with God's Word it is Baptism.
Why would the baby be in need?
Because infants are under the curse of sin. God has provided the cure.
Provided the cure? Infant baptism it is? In the context of this conversation, where else can you be going with definition of the cure?
Water without God's Word is water, with God's Word it is Baptism.
Then we may as well baptize puppies too, pronouncing the words over them as some form of incantation.