"I have two children and one is a son born on a Tuesday." If it were about the phrasing you could assume the other child is a daughter - if he were to continue

"I have two children and one is a son born on a Tuesday." If it were about the phrasing you could assume the other child is a daughter - if he were to continue ... "The other is also a son, born on a Friday" it would just sound strange. Perhaps it is all down to his turn of phrase :)