Brethren, be not children in understanding: howbeit in malice be ye children, but in understanding be men.
Brethren - an appellation to conciliate their favourable attention.
Children in understanding - as preference of gifts abused to non-edification would make you (cf. Matthew 10:16; Romans 16:19; 1 Corinthians 3:1; Ephesians 4:14). The Greek х fresin (G5424), not elsewhere], "understanding," expresses the will of one's spirit: one's mindedness (Romans 8:6), as the 'heart' is the will of the 'soul.'
Men - full-grown. Be child-like, not childish.