The Belgian Senate is to loose its three royal members upon the retirement of King Albert II. Prince Philip, prinsess Astrid and prince Laurent will no longer be entitled to their seat, by law reserved for the children of the monarch as they cease to be just that, children of a reigning King.
Philip of course will become himself King of the Belgians, in itself reason to give up his seat in the Senate. The new King's children are still minors, so they do not qualify. The new presumptive heir to the Belgian throne, Princess Elisabeth, is only 11 years old.