Excursions can be much cheaper booked on your own...besides, you can be on your own schedule then. Captain Marvin's is great! There are other companies that offer similar things, but his is the best.
Your biggest cost savings is in the 10-12 age range.
DCL considers your child an "adult" for cost purposes on excursions at 10, whereas most private vendors start adult fees at 12 or 13.
The dolphin encounter is just that...you get in the water and the dolphin swims close enough for you to touch. It is NOT a "swim with the dolphin" thing. If you have the time, a much better deal is the dolphin swim at Discovery Cove in Orlando (a sister park to Sea World). It is about $225 per person for the day, but that is an "all enclusive" day....parking, and entry photo, lunch, wet suit, dolphin SWIM (limited to 30 minutes in the water by FL law)...plus unlimited time in their lazy river, coral reef, stingray pool, aviary, etc....and includes all animal food.
On the Western, Chankaunab park offers something for everyone. A taxi is $20. You do need cash for admission, but once in, all "stuff" can be charged. It is another "much cheaper to do on your own" than thru DCL.
We really aren't crazy about ports on the Western...and frequently stay on the ship! (veteran of 10 Magic cruises....)