I agree with Donna!
First and foremost, it's important to book with an agent, preferably a Cruise Specialist, who can discuss all your options and help you find the perfect cruise.
Keep in mind that like I always say, what one person loves, another will hate. So just because her grandparents loved Royal Caribbean, does not necessarily mean that your son and his wife will.
As Donna has stated, some of the other cruise lines may be more well-suited for them and there are some great ones out there.
It's important to work with an agent who will ask alot of questions and help you find a cruise that meets your requirements based on their lifestyle, needs, and budget. They are all good, but each offers something different and this is such a wonderful celebration, you need to get them a wonderful cruise to match.