Tom's Midnight Garden (1999)

When Tom's brother Peter gets measles, he is sent to stay with his Uncle Alan and Aunt Gwen in a flat with no garden and an elderly landlady, Mrs Bartholomew, living upstairs. Because he may be infectious, he is not allowed to play outside and feels lonely. Without exercise he is less sleepy at night and when he hears the communal grandfather clock strangely strike 13, he investigates and finds the small back yard is now a large sunlit garden.

Unfortunately the movie Tom's Midnight Garden is not yet available on HBO Max.