Different Episcopalians will have different perspectives on this matter, but in the creed we do recite that there is "one baptism for the remission of sins." This is not only for past sins, but for the sins of the whole lifetime. This doesn't mean that one is given a free pass to do whatever one wants because the sins are already forgiven, but that when one falls into sin one may renew not only one's baptismal vows, but the graces and promises that God makes to us in baptism. Because an infant will likely sin in their lifetime their baptism is no less for the remission of sins than anyone else's.
Now some infants (such as those likely to die) may not have a chance to sin. Nevertheless we believe there are other graces in baptism such as incorporating the child into the church. In our understanding we see no reason we should not incorporate them into the covenant with God -- this itself is a grace apart from anything else.
We don't exclude children from the covenant, nor the severely mentally disabled for that matter. They, too, may receive baptism and Holy Communion.
This post is a pretty informative view of how many Episcopalians would view infant baptism:
Infant Baptism: What the Church Believes | Antiochian Orthodox Christian Archdiocese