After hearing the apostle Peter's sermon on the Day of Pentecost in Acts 2, many people asked him what they should do. He answered, "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call" (verses 38-39).
In this passage, we see that God calls people to Himself, expecting them to repent and be baptized in order to become part of His family. Realizing that people cannot come to God until they are called (John 6:44), we naturally wonder about the children of those God calls. The passage in Acts 2:39 answers this question. Children of those whom God calls are also extended the promise of receiving the Holy Spirit if they repent. Because this calling, or invitation to have a relationship with God, is offered to these children, they are called "holy" in 1 Corinthians 7:14.
Of course, young people must choose to respond to the calling they receive through their parents. God will not force anyone to respond. If you'd like to study this subject in greater depth, we suggest you read the United Church of God study paper, "Are Our Children Called Now?"