I too was on the Tortola cruise. This was my family's first time to any of the islands, but my DH and I are pretty confident travelers, so we did everything on our own. We also did this with a 4 yrs little girl.
The Baths: We got off the ship and just as someone said previously, there were very sweet ladies providing information about the island and how to get where you wanted to go. We had an idea of where we were going, but wanted to verify. The ferry to catch to Virgin Gorda was about 4 blocks to the left of the ship once you disembark. Now there lots of taxi drivers, but just keep saying, no thank you. They were all very nice and didn't push. Once you get to the end of the blocks, there's a stop there (with a stoplight to come soon), turn left and to Speedy's. They have ferry's leaving about every hour and half. Speedy's has a website for times (but once there we saw they had additional times). We bought $30 p/p roundtrip to Virgin Gorda and discounted Speedy's taxi for $8 p/p roundtrip to the Baths. Our 4 yr was free for both the ferry and taxi. They have different type of ferries. The one we took there and back were both very nice and looked like local business commuters. It was about 30/45 minute ferry. Met our taxi shuttle and headed to the Baths. Don't attempt to walk it - it's way too far. Once at the top of the Baths, we hiked down the easier path to the Bath Beach, went through the caves and came out onto Devil's Bay Beach. It was beautiful. Just remember, if you do this route, do all your potty stops, food eating and anything else before you start through caves. Because there is nothing on Devil's Bay Beach. We carried everything with us. DH and I each had backpack. We had a great time exploring the caves and spending time on the beach. It was beautiful and happy we did it. The best snorkling was on the left side of Devil's Bay Beach near where the bouys are tied up. Because we did this on our own, we spent a few hours on the beach and then up the Devil's Bay Beach path. Yes, it was a more treacherous patch and longer, but we just took our time and our little had no problem with any part of it. She even commented as how beautiful it was with cactus and lizards. We wore our Keene sandals (hiking, walking, and water shoes). No socks needed, but they also were not wet before we started our hike up. Once up at the top by the restaurant Tops of the Baths, there is a little foot wash just outside the restaurant. Not much to it, but it worked. Oh, and there is an additional little sandwich/ice cream shop at the top in addition the restaurant. We didn't even notice it until we were about to leave. Anyway, our speedy taxi was there on time and we were off back the ferry back to Tortola. We didn't really see Tortola, but maybe next time.
Sorry for the detail, just wanted to share for those of you who wanted to do the Baths on their own. And yes it is possible to do with 4 yr. old. She walked it all except for our walk back from the ferry to the ship. She got to sit on DH shoulders, she was tired. If you have any questions, I'll answer as best as I can. Enjoy your cruise.