The film script indicates that not only do we (the audience) not see the execution, but that the trio can't see it either. Their eye-line was obscured by a stand of trees.
The crows SHRIEK excitedly and the Executioner separates from the others, disappears behind
a LOW STAND OF TREES.
DING!
All goes very still. The wind loses its voice...
DING!
Sunlight kicks off the axe as it rises over the trees,
INTO VIEW...
DING!
The ax hangs seemingly forever, etched against the sky, then drops from sight. A SICKENING CHOP! fouls the breeze and Ron and Harry stare toward the trees in numb disbelief.Ron and Harry stare toward the trees in numb disbelief.
In the book, Hermione. Ron and Harry exit the scene (stage left) and are quite far away when the execution takes place. They hear it but they don't see it.
They walked forwards; Harry, like Hermione, was trying not to listen to the rumble of voices behind them. Ron stopped again.
‘I can’t hold him – Scabbers, shut up, everyone’ll hear us –’
The rat was squealing wildly, but not loudly enough to cover up the sounds drifting from Hagrid’s garden. There was a jumble of indistinct male voices, a silence and then, without warning, the unmistakeable swish and thud of an axe. Hermione swayed on the spot.
‘They did it!’ she whispered to Harry. ‘I d-don’t believe it – they did it!’
Harry Potter and the Prisoner of Azkaban - CHAPTER SIXTEEN – Professor Trelawney’s Prediction