One of the points of Psalm 8 is that even the smallest, weak, and frail of God’s covenant is sufficient to undo the unfaithfulness and the faithless ones in Israel. Jesus picks up on the same theme in the Gospels when he rebukes the Pharisees for assuming infants to be an unnecessary part of Jesus’ mission.
If the psalm follows the creation pattern, then it entails progression. There can be no day 6 without day 1, proving then that infants are part of the creation and re-creation story. Infants are seeds, but seeds that will grow to reach the maturity of day 6.
The main point of verse 3 is that babies are potent soloists in God’s creational concert.