I'd take a Na Pali Coast boat tour over the helicopter and have done both. The boat tour is about 5 hours vs 50 minutes on a heli. The one problem is going to be the 2 year old, Captain Andy's large catamaran out of Port Allen only allows age 3 or 4 and up so you might have to get a sitter.

I thought the train ride was only "okay." For me, the highlight of the train ride was the part where they stop and you see the animals, and I'd think the kids would enjoy that.

I've never done a helicopter tour, but the Na Pali boat ride is INCREDIBLE. But be forewarned that it is not a calm, gentle ride. At times it can be rough. So if anyone in your party gets motion sickness they should take pills in advance.

For me, Kauai is not about paid activities. It's more about enjoying the beaches and scenic beauty. For me, there are "must-sees" -- not "must-do's." Go to Hanalei Bay. Drive to Ke'e Beach (at the end of the road). Spend a day at Anini Beach. These are all very close to where you'll be staying. And although it will be a long drive, Waimea Canyon is a must-see.

The pool is beautiful at the Wyndham Bali Hai and I suspect the kids will enjoy that more than any paid activity.

Enjoy!