Anachronisms

Elspeth Dudgeon who played Susan Sowerby was 78 years old, she would therefore have been well into her 60's when she gave birth to Dickon.

Continuity

In the opening title sequence of the movie, someone unlocks the door to the Secret Garden, and pushes it open, inward from the right side. In the rest of the movie, the door opens inward from the left.