From my experience, the sand fleas seem to be more of a problem on the beach after sunset when it gets cooler. Several people highly recommended Avon's SKIN-SO-SOFT Bug Guard PLUS Insect Repellent Moisturizing Lotion as the best product to prevent sand flea bites. Since I am not comfortable using DEET on my little ones' skin, I really like that it is DEET free, yet still works great. It comes in a lotion, spray, or individually wrapped towelettes. I used to have a neighbor that sold Avon and I would order through her, but now I just order it from Amazon.
I always pack the towelettes just because it's easier to travel with, and make sure to wipe them all around our ankles before sunset and we haven't had any problems. For some reason they seem to love attacking the ankle/foot area.