We did the hike on our NYE cruise at the end of 2014 (you will love spending NYE on the Fantasy, by the way -- our best NYE ever!). At the time, our daughters were ages 10 and 8. I didn't think the hike was strenuous at all, though my husband and I still laugh about our then-8-year-old's constant grumbling during the hike ("Nobody asked me if I wanted to go on this hike!" "Who signed us up for this?" Etc.). Our 10-year-old made it with no complaints.
Although we thought St. John was gorgeous, the clear highlight was Honeymoon Beach, and we just didn't have enough time there. The ferry ride over from St. Thomas was really quite miserable (very rough seas - several sick passengers). It took a little while after we got to St. John to even start to feel okay again. Much of the hike is walking in the rain forest with no real views of the ocean and not much in the way of wildlife. When we finally got to Honeymoon Beach -- which was breathtaking -- I think we only had 20-30 minutes there. There was a snack bar, and I spent basically my whole time at the beach waiting to order and then waiting for the girls' smoothies to be ready. This was clearly not the way I wanted to spend my 30 minutes in paradise!
It was an okay excursion, but I don't think I'd recommend it unless there is just nothing else that appeals to you. If you do the excursion, be sure to pack snacks, because you won't have a chance to eat a real lunch.