"This is Abraham and Sarah's boy."
Translation:זה הילד של אברהם ושרה.
No, only שייך means "belongs to". של means "of".
הילד של אברהם - the boy of Abraham, or more naturally Abraham's boy
הספר של אברהם - the book of Abraham, or Abraham's book
הספר שייך לאברהם - the book belongs to Abraham
הציפור שייכת לאברהם - the bird belongs to Abraham
So, שייך requires ל. Also, שייך changes to שייכת if the object is feminine - in these two sentences, a book ספר is masculine, so it's שייך, but bird ציפור is feminine, so it's שייכת.
In this case I think it would be correct to translate it both with and without the ה, but it has different meanings. It's easier to see the meaning if you translate it literally with "of" for "של":
- Meaning 1: זה ילד של אברהם ושרה = This is a boy of Abraham and Sarah (implying that they have several children and this is one of them).
- Meaning 2: זה הילד של אברהם ושרה = This is the boy of Abraham and Sarah (implying either that they have only one boy, or that you are referring to a specific boy of theirs that was mentioned earlier).
I think the original English sentence can be interpreted as either of these.