My child was hit in the eye with the water from garden hose at full blast. what to do for his eye?

Likely no harm: The water itself is not harmful. Depending on the force of the stream, some damage from the blunt trauma may have occurred. If the child is old enough to assess his vision, you can check that he sees the same clarity from one eye to the other, and look for any redness of the white of the eye, or hiding of the colored part of his eye. If any doubt that the affected eye is normal, see an eye md asap.