I would highly recommend taking a scuba diving course with your kids. They can start as early as 8 years old, however in order to be classified as open water autonomous divers you need to wait until they are 10.

I don't know where you are located, but PADI open water course outside of the US is about $200 and takes 3-4 days.

https://www.padi.com/help/scuba-certification-faq

Its a skill they will have for life and it will be something you will talk about for years.

The only downside is that once you start diving all of your vacations become diving trips.

As far as resources are concerned, go to a used book store and pick up tropical reef animal (coral are animals) identification books. They are great coffee table reading and are effectively real life Pokedexes.