The SCNR pdf also has a great picture showing the bi-valve of the creature. If patient, you can watch the coquinas burrow out of the sand and extend their valves. They feed on micro-organisms like phytoplankton and bacteria. They are fed on by birds and fish.
Given the nature of the waves, this video isn't great, but I did manage to catch a few of the coquinas burrowing mid-wave if you look towards the bottom of the screen. I could watch these for hours, though Conor was interested for a few minutes — he did seem to get a bigger kick out of the birds that were feeding on the clams.
While in Sarasota, we had the chance to visit the Mote Marine Laboratory and Aquarium — I recommend it as it's mostly enclosed and I found it to be very kid-friendly. It's small but has a great collection, including a shark tank. Eels, clown fish, jellyfish, they're all featured in a space easily walkable by the 3-5 year-old crowd, and it's got informative exhibits for the animals in its tanks.
At the Mote, I also found a great book in the gift shop entitled Awesome Ocean Science by Cindy A. Littlefield. She's got a fun experiment on page 40 that shows how undertow is created. I won't go into detail here but I recommend looking it up as it can answer questions from the wee ones on why their feet sink in the sand along the shore.
I also really liked her idea (on page 47) for creating a simple underwater viewer for tidal pools. A cylinder (like a coffee can with a good field of view) is covered by plastic wrap on one side and secured by a rubber band so that that end can be lowered into the tidal pool to provide a clear view of the creatures residing there. The beach we were on didn't really recede into tidal pools at low tide, but certainly if you have a chance, these provide a great opportunity to view nature. In the pools you can generally find crabs, small fish, urchins, and even the occasional starfish. Of course, the idea is to observe these creatures without removing them from their habitat and without hurting them. It's a great chance to commune with nature and to show kids the wonders of ocean life.