Main Point: The sign of the covenant of grace has belonged to believers and their children from the time of Abraham and into the time of the New Testament church.
Outline:

Baptism is for Believers (Rom. 4:11; Gal. 3:7; Acts 2:41)
Baptism is for Their Children

a) The nature of the covenant of grace is not that its members are necessarily saved (Gal. 3:7; Jer. 9:25)
b) Children were seen as part of the covenant from the beginning (Gen. 17:10; Acts 2:39)
c) We see household baptisms as a norm in the New Testament and in early church history (Acts 16:15; 33-34; 18:8; 1 Cor. 1:16)