The Boy With The Bronze Axe, Kathleen Fidler.
This is the story of life nearly 3,000 years ago. Kali and Brockan are in trouble. They have been using their stone axes to chip limpets off the rocks, but they've gone too far out and find themselves trapped by the tides. Then, an unexpected rescuer appears, a strange boy in a strange boat, carrying a strangely sharp axe of a type they had never seen before.