CoD 1.17

Article 17: Children of Believers Who Die in Infancy

We must judge concerning the will of God from his Word, which declares that the children of believers are holy, not by nature but in virtue of the covenant of grace, in which they are included with their parents.1 Therefore, God-fearing parents ought not to doubt the election and salvation of their children whom God calls out of this life in their infancy.2

References

Genesis 17:7 (ESV):
"And I will establish my covenant between me and you and your offspring after you throughout their generations for an everlasting covenant, to be God to you and to your offspring after you."
Isaiah 59:21 (ESV):
"And as for me, this is my covenant with them, says the Lord: my Spirit that is upon you, and my words that I have put in your mouth, shall not depart out of your mouth, or out of the mouth of your offspring, or out of the mouth of your children's offspring, says the Lord, from this time forth and forevermore."
Acts 2:39 (ESV):
"For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself."
1 Corinthians 7:14 (ESV):
"For the unbelieving husband is made holy because of his wife, and the unbelieving wife is made holy because of her husband. Otherwise your children would be unclean, but as it is, they are holy."