Wolfmueller, Has American Christianity Failed, 131-133 (probably the best, succinct explanation I’ve read)
Here Jesus is greatly displeased with His disciples for forbidding the little children to come to Him. So also is He greatly displeased with anyone who will forbid little children. Dare we deny them Baptism, then? For to be baptized is to come to Him. Yet someone will say, they cannot believe, for they cannot yet reason. Ah, but is reason that which produces faith? Certainly not! Rather, it impedes faith, which is why Jesus says here, whoever does not receive the kingdom of God as a little child will by no means enter it. So then little children may not be denied Baptism; rather, let Baptism be denied to all who will not become like little children. Let faith be pure and childlike, unsullied by the treachery of reason, as St. Peter also says, As newborn babes, desire the pure milk of the word. And again, David says in the Psalms, out of the mouth of babes and sucklings hast thou ordained strength. For as Jesus took up the children in His arms and blessed them, so will He bless all who will become little children in His sight, returning daily thereby to the simple, baptismal faith of infants. (Burnell F. Eckardt Jr. Every Day Will I Bless Thee. Sussex, WI: Concordia Catechetical Academy, 1998, p. 114.)
Gerhard, Heaven, § 46.4: Just as after the fall salvation comes to no one without Christ, so also no one can be saved without the faith that apprehends Christ. This must be brought against those who ascribe salvation to the more sound heathen who yet lack faith in Christ and who deny that baptized infants believe and yet concede that they are saved.