Hello
chiefwalkenhorst,
To try to answer you in a nutshell: the Reformed hold that the Covenant of Grace (CoG) is the overarching covenant of both Old and New Testament times, with differing administrations and signs/seals. Thus the command to Abraham, Gen 17:7,10,11,14, to put the sign of the covenant on his male children — the failure of which cuts the child off from the covenant — carries over to us in this dispensation of the CoG, whereby females are also given the sign (Col 2:9,10,11,12,13) :
For in him [Christ] dwelleth all the fulness of the Godhead bodily. And ye are complete in him, which is the head of all principality and power: In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ: Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead. And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses...
We administer the sign of the covenant in obedience to the God of the covenant, still being children of Abraham : "And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise" (Gal 3:29). And as did Abraham with the sign of circumcision, we baptize all, for the sake of the elect among them.
As regards Heb 6:4,5,6, this pertains to those born and raised in a godly church under the power of the Gospel who nonetheless are reprobate, even though the Holy Spirit wrought deeply upon and within their hearts, but were that ground and seed "which beareth thorns and briers [which] is rejected, and is nigh unto cursing; whose end is to be burned (Heb 6:8).
See also
this post from a recent thread here on regeneration or "faith" in infants.
(P.S. You should fill in the signature for your PB account, so we know who and whence you are.) By the way, welcome to PB!