How Can a Sign and Seal of the Covenant of Grace Be Upon an Unregenerate Child?

Consider this definition of baptism:

Baptism is an ordinance of the New Testament, ordained by Jesus Christ, to be unto the party baptized, a sign of his fellowship with him, in his death and resurrection; of his being engrafted into him; of remission of sins; and of giving up into God, through Jesus Christ, to live and walk in newness of life. – LBC 1689, Chapter 29

And this one:

Baptism is a sacrament of the New Testament, ordained by Jesus Christ, not only for the solemn admission of the party baptized into the visible Church; but also to be unto him a sign and seal of the covenant of grace, of his ingrafting into Christ, of regeneration, of remission of sins, and of his giving up unto God, through Jesus Christ, to walk in the newness of life. Which sacrament is, by Christ’s own appointment, to be continued in His Church until the end of the world. – WCF, Chapter 28

Now consider this question posed by one of my elders:

How would you apply these definitions to an infant child?

Finally, Consider this verse:

In Him [Christ], you also, after listening to the message of truth, the gospel of your salvation–having also believed, you were sealed in Him with the Holy Spirit of promise, – Eph. 1:13

According to Scripture, the Holy Spirit is the One who seals the elect, AFTER they have believed. So, what seal is being applied to an unbelieving infant who has been baptized?