Is seth the first child of Adam?

Yes, Seth is considered the third son of Adam and Eve in the biblical narrative. According to the Book of Genesis, Adam's first two sons were Cain and Abel, with Seth being born afterward as a replacement for Abel, who was killed by Cain. Seth is significant in the lineage of Adam, as he is seen as the ancestor of Noah and, ultimately, the line leading to Jesus in the biblical genealogies.