Welcome to the Disboards!
Keeping the dirt out of a tent is not easy at the Fort. The coquina/sand sticks to shoes and feet. A couple of tips:
-If possible, set up your tent so that the entry sits right on the edge of the concrete slab. If you can do this, then no one will be walking through the sand to get in the tent.
-Set out mats for everyone to wipe their feet before entering the tent. If possible, have everyone remove their shoes before entering the tent, or set up a place right inside the tent for this. Have a mat outside to wipe feet, and then another inside to stand on to remove shoes. Back in my tent camping days, I had the kids wear flip flops around camp (which is fine at the Fort, but obviously wouldn't be OK at some place with a lot of rocks and such), and this made it easy to remove their shoes before coming in to the tent.
These two tips should cut down on the sand problem. But, even if you do this, that pesky sand will get in your tent. The only thing that you can do about it is to be sure to have a whisk broom and dust pan, and use it often. I had a friend who used a really fine brush instead of a regular straw type whisk broom, and that seemed to really help with the sand.
Have fun!
TCD