Once they are dry, you can gently brush it off with a soft towel or your fingers and it won't scratch the skin.

Rub dry sand gently on the wet sand. Works a treat.

Tell them to run back into the sea (obviously tell them not to go in too deep), the water washes it off and you just need to wrap a big towel around them.

Rub talc on them and the wet sand will then brush off.