As Prince Sverre Magnus does not have a prefix himself - his children would hardly get a HH 😛 in fact I even see it as highly unlikely his children will get titles.

Remember also that Queen Sonja - the spouse of the previous reigning monarch - has been stripped of her prefix as Her Majestry.

So therefore I think the line will be: prefix for the reigning monarch + spouse, for the heir + spouse and for their eldest child + spouse.