T.T.
I don't think ingesting anything other than food is good for them, but I don't think it really do a lot of harm. As long as it's not a lot constantly. The first thing my son did was eat sand as soon as we took him to the beach, it happens. It was on food, his cup, everywhere, and he was fine. You just have to keep a close eye on them, especially with the shells and rock, HUGE choking piece.
Enjoy the beach with the babies,
T.