(The Internet, as is its wont, disagrees. Suffice it to say, the book is not long, and variations in number from place to place probably depends on the edition, and whether forewords, etc., are included in the word count.)
Nevertheless, that seems really short.
“Ugh!” said Susan from the other side of the Table. “How beastly! There are horrid little mice crawling over him. Go away, you little beasts.” And she raised her hands to frighten them away.
“Wait!” said Lucy who had been looking at them more closely still. “Can you see what they’re doing?”
Both girls bent down and stared.
"I do believe!” said Susan. “But how queer. They’re nibbling away at the cords.”
“That’s what I thought,” said Lucy. “I think they’re friendly mice. Poor little things – they don’t realise he’s dead. They think it’ll do some good untying him.”
It was quite definitely lighter by now. Each of the girls noticed for the first time the white face of the other. They could see the mice nibbling away, dozens and dozens, even hundreds, of little field mice. At last, one by one, the ropes were all gnawed through.
The sky in the East was whitish by now and the stars were getting fainter – all except one very big one low down on the Eastern horizon. They felt colder than they had been all night. The mice crept away again.