Welcome aboard, Anthony! I'm happy to see you've come back with another question before you set sail!
You didn't mention which port you were looking at when talking about the rainforest hike, but I'm going to guess you meant the Mountain Top Downhill Rainforest Trek in St. Maarten. That says that the age begins at eight. I wish I had better news, but the Port Adventures operators are very strict about the age limits they list.
I'm not sure when you sail, if it is before or after your daughter turns six. But, if it is after she is six, you might consider the
St. John Eco Hike which you can read about on Disney Cruise Line's website. I'm assuming you're on the Eastern Caribbean itinerary, so you would probably be stopping at St. John.The minimum age on this Port Adventure is six, so this could be a nice hike alternative.
If you're set on a rainforest hike for the family, you might look into non-Disney sponsored excursions at the ports. If you google the port and the type of excursion you're interested in, perhaps you'll find something similar that has a lower age requirement. Keep in mind though, that if it's not a Disney-sponsored Port Adventure, you are on your own to be sure you get back to the ship in time.
Good luck planning, Anthony, and let me know if I can help with anything else before you sail!
Happy cruising!