Take it in turns to write or draw something in the sand of a sand tray which you are sorry for.
Encourage each person to say sorry to God and ask Him to forgive them. This could be 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.