We booked this boat tour to see the Napali Coast with out 2 1/2 year old. Most boat tours require children to be 5 or 6 to board, (and I think this one mentions it as well), but just call them up anyway if you're thinking of booking a ride with younger children - this one allowed it (there were 3 other children "underage" on our ride, and one pregnant woman). Highlights were animal sightings (dolphins) and being able get see the Napali Coast cliffs up close and personal in the boat.
I'm giving this tour 3 stars because of the length of time we had to be on the boat (about 5 hours) and the brevity of our allotted snorkeling time (about 30 minutes). And the cost - $140/person, $95 for children. (We signed up last minute for a morning tour and they comped our son - probably because he was too young). We went in early December, but the weather was beautiful and the waters were relatively calm (my wife who easily gets sea sick didn't have a problem). The coastline is spectacular and the captain/crew of the boat were very friendly, helpful, and informative. I'd easily give this 5 stars if it were a 1-2 hour tour with 1 hour of snorkeling, but that doesn't seem to be how they do it out here - at least not from Port Allen. The length of the tour could be challenging for the toddler set, but our son survived without any meltdowns (except for when we took him into the water with us to snorkel and looked like malicious parents as he complained and cried about the water being too cold). He just passed out afterward for the last 30 minutes of the boat ride.
This tour, as with most boat tours of the Napali Cost, leaves from Port Allen, about a 20-25 minute drive from the Poipu area in the southern part of Kauai.