We've both traveled, seen many places and done many things. Zip-lining, deck-puking, sun-burning, midnight buffet, late shows, etc, etc, etc. We've had our 20s (and most of our 30s) to do whatever we wanted. Then we chose to have children. Now, a cruise that would be relaxing... but boring because we've done it all 10 times... is a totally different experience.
I would never give up the memory of my son getting a dolphin snout in his mouth because he hadn't learned to pucker yet. When they said "kiss the dolphin" he gave an open mouth kiss. I would never give up the memory of the look of wonderment and joy on my daughters, face when she got to pet a turtle larger than she was. With 4 cruises under our belts, I have dozens of those memories. As the commercial says: "priceless"
I won't lie and tell you that it's easy. It isn't. I won't tell you that you can do anything you want to. You can't... well, you might have better luck since you have 4 adults and one child. Generally though, there are trade offs to be made and that is for you to weigh.
As far as the suggestion(s) that a toddler wouldn't enjoy a cruise... let's see: almost unlimited time with parents, one or both of whom work the rest of the year... a literal buffet of food choices everyday... crew members and passengers fawning over them... finally a decent opportunity to wear a tuxedo... endless entertainment in the form of people-watching, bells and whistles everywhere, etc. What's not to love?
Toddlers don't "get" the cruise/travel experience, but they most certainly have a ball on a cruise ship. Mommy and daddy, perhaps not so much. But that is what we signed up for, and IMHO the permanent memories far outweigh the temporary difficulties.