Dr. David Scaer makes a point that should have been obvious to me, but wasn’t, in his piece “Flights from the Atonement” in the latest Concordia Theological Quarterly (July 2008): by denying the ability of infants to have faith, they thus deny justification by faith. Children who have not yet reached the maturity to make a rational decision for Christ are saved on account of their human birth, not the birth from above (John 3). The following is from pp197f, with my emphasis added:

Like Roman Catholics, the Reformed do not see justification as the one chief doctrine. Evangelicals who stand in the Reformed tradition may share with Lutherans a verbally identical definition, but in understanding faith as a conscious rational decision of which only the intellectually mature are capable, their definition is compromised. Since infants and young children cannot believe, their birth within a Christian family – and not faith – gives them a place within the covenant. Prima facie justification by faith is denied. The Evangelical or Reformed definition of faith which does not allow the fides infantium compromises their understanding of justification of faith and calls into question other aspects of their theology. Only that faith which is pure receptivity responding in trust to Christ qualifies as the sola fide by which sinners are justified. Self-reflection does not belong to the faith which justifies.