Lisa,
If I remember correctly, the cost was maybe $30 per person. We had a 1 year old who I believe was n/c. I don't remember a difference between adult & child. I think also it was a 2 hour cruise but ours was cut short (time wise) because of the rain. Although it sounds expensive, the experience more than offset the cost for us. My son could have actually touched a couple of dolphins if we had allowed him -- they were that close. We were able to call when we arrived and select the day that we wanted to tour. I believe the one through Disney runs multiple times a time. We also had a wonderful experience seeing dolphins when we went on the beach combing outing also arranged through Disney. We went with the same guide as on the dolphin cruise. I believe that was $50 per person (kids included). Our 1 year old was no charge again. We were taken to a private island to hunt shells. On the way back, a fishing boat was cleaning their catch and the dolphins were swarming all over the place. We pulled up next to the big fishing boat and were talking to the fishermen. I doubt we'll ever be able to have such a natural experience again. It was terrific. We will probably do the beach combing again in July because if we had to choose between the dolphin cruise or beach combing, I'm not sure we could. Also, on our cruises, we never felt rushed. Especially with the beach combing one. When we pulled up by the fishing boat, we watched and watched. Pushed it enough that we almost didn't make it back to Disney before dark. Have a great time!!