Think of something which you are sorry for. Then take it in turns to write or draw that thing in the sand tray (or the beach!).
Encourage each person to say sorry to God and ask Him to forgive them. This could be done silently as they draw or write or spoken aloud. Shake the tray to make whatever was drawn or written disappear.
You might like to say: Jesus forgives you and wipes this away completely.
If you are near the beach, you could do this down by the wave line, letting the waves represent God’s complete forgiveness.