"For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." (Acts 2:39) Three observations:
1) Notice that the promise is monergistic - it is to those whom the Lord calls - that's how the promise is phrased. It's not to "even as many as shall work really, really hard."
2) Paedobaptists sometimes quote this passage incompletely as though it just say "unto you and to your children." It says more than that, and the "even as many as the Lord our God shall call" is definitely key.
3) Still, I haven't heard my Reformed Baptist brothers provide an adequate explanation for the reference to children, if not to suggest that God is going to continue dealing with families as families. This looks like the kind of thing we see several times in the Old Testament - now with an expansion to those who are not Jews. That's not the central point of the verse, but it seems to be the most obvious reason for the reference to children - a passing reference to the fact of familial treatment that God applied up to this point both with respect to Israel and the nations.