Let the child
At such pronouncing by the second by sound after it it will be heard shch.
Pay attention of the child that after a sound h is heard other sound similar to noise of a sea surf.
Let the child as it is possible pulls this second sound more long.
Further the pronunciation of the sound going after h shch has to to interrupt short pauses as though one wave, rustling, runs on the coast for another.
Pauses should be extended so that after a combination chshch sounded actually isolated sounds following one after another shch as though waves run on the coast less frequently..