There is much more to Robbie's of Islamorada than just the tarpon feeding off the dock but for affordability and the fun factor this wins out as one of the top things to do with the kids in the Keys.  For about $4 you can gain access to the dock ($1) and get a bucket of bait fish ($3) to feed the tarpon.  Your kids will be delighted and amazed by these creatures and get a real kick out of feeding them.

John Pennekamp Coral Reef State Park is a Florida state park located between 2 bodies of water: Largo Sound and Tarpon Basin on the Atlantic Ocean side of Key Largo. The park consists of about 70 nautical miles and encompasses several types of habitats.  The mangrove swamps, upland areas, and hammocks offer some interesting land exploration opportunities on well-maintained trails. The main draw for the park are its spectacular coral reefs which can be viewed by glass bottom boat or on snorkeling and scuba diving expeditions.

Sundiver Snorkeling Tours has been providing an expert snorkel experience within the John Pennekamp State Park, or National Marine Sanctuary, since 1972.  Their Coast Guard certified staff are experienced with families, kids and first time snorkelers.  They visit one of dozens of shallow reef sites each time they go out.  You can snorkel several times with them and experience different areas each time.  The price of the tour includes enrtrance fees, ice cold water and transportation.