How do you say seventh child in Hebrew?

There isn't a single word in Hebrew for "seventh child." You would say it descriptively:

הילד השביעי (ha-yeled ha-shvi'i)

* הילד (ha-yeled): the child

* השביעי (ha-shvi'i): the seventh

Therefore, the whole phrase means "the seventh child." If it's a girl, you'd substitute ילדה (yeledah) for ילד (yeled): הילדה השביעית (ha-yeledah ha-shvi'it)