νήπιος. This is a frequent metaphor in St Paul, who also contrasts “babes” (νήπιοι) with the mature (τέλειοι), Gal 4:3; 1Co 2:6; Eph 4:13-14. We are only to be “babes” in wickedness (1Co 14:20).
λόγου δικαιοσύνης, i.e. the Scriptures, and especially the Gospel (see 2Ti 3:16; Rom 1:17, “therein is the righteousness of God revealed”). The Hebrew צְדָקָה has almost the sense of ἀλήθεια.