SPOILER
The sister of the boy who befriended the Borrowers later went to the house where he met them. She found Arrietty's journal, and all the letter Es were written the way her brother used to write them.
The implication is that the boy wrote the journal. No Arrietty, and no Borrowers.
The sister - who is in fact, as an adult, the narrator - also relates how she herself left things for the Borrowers to find, which they duly took away. This is the bit that bothers me most, because while she might not know whether the boy's story was made up or not, she does know whether her own part was. So if she knows there are Borrowers, why cloud the issue; and if she knows there aren't, why dress the story up as true and then embellish it with a lie?
I know it's "only" a children's book, but seriously, I was actually quite invested in it. There was a terrible poignancy in the account of the lonely boy abandoned to a drunken aunt and indifferent servants, left to play alone. You knew from the outset that he later dies young in battle, and I warmed to him because of the fate he had in store.
I also liked the fact that the relationship with the Borrowers wasn't a cutesy-cutesy Narnia-esque aren't-they-sweet load of twaddle. To begin with he is actually pretty cutting towards them, telling them that what they do is simply stealing. But later, in his conversations and deeds, he is sometimes the neglected child ("You could have read those to me, but you never come") and sometimes the parent he needed someone to be. He brings them things they need, and tries to protect them.
When he's locked in his room, he's humiliatingly cut down to size again and it actually prefigures the end of his short life.
It actually reminded me a bit of the early life of Douglas Bader, the Spitfire pilot who lost his legs and came back from it. His parents went to India without him when he was two. He didn't see them again till he was about five, and not long after that, his father was killed in World War One. With that amount of emotional neglect, it's not surprising he turned into a tough SOB.
So yeah, I found The Borrowers really quite moving, until she cocked it all up.
It reminded me of Green Knowe, where you can guess, but you don't ever really know for sure, that the child who sees the ghosts is probably dead himself.