This is a photo of the best of my five attempts. It actually looks better in this photo than it does in person.
I had a number of problems over the course of five of these. I had cracked plaster, feet with no toes, sand that was too dry then sand that was too wet. I cannot tell you how to successfully do this but I will tell you the theoretical process. I will never, ever attempt this again so if you try it and are successful send me a note giving some hints and I'll post them.
You need sand, plaster, a clear sealant spray, and a picture frame.
Remove the glass from the picture frames and put the back of the frame in backwards.
Fill the frame with sand and have your kid(s) make footprints. Here's a photo of that step.
Smooth out the sand around the prints and cover in plaster.
Let it dry completely then flip frame over carefully to remove plaster and sand.
Brush excess sand off and spray with sealant. You can also add shells to make me really jealous.
All in all, I wouldn't recommend this at all if the end result wasn't so cute. It's messy and the only interactive part for kids is making the footprint, which isn't much fun. I won't be trying it again but good luck to you if you try it!