Digdilem ably sums it up in their review. I watched DoD because I knew Dartmoor and was pleased to recognise various locations. The three child actors were adequate. (The two lads went on to have short acting careers;this appears to have been the girl's only appearance.)
The dog was very good!
I immediately recognised Patricia Hayes in one of her trademark characterisations and initially wondered if that really was Barry Foster in a lightweight production. It was, and at a time when he was immediately recognisable by many on the big and small screen, not least as Van de Valk. Perhaps he took part as a favour to the producer, not that a few days in Devon would have been much hardship.
I'm glad that I watched it.