Looking for a (probably)kids' TV episode/movie about a mysterious neighbor, two ...

I've been trying to find this for years and it's driving me crazy.

I'm looking for either a live-action TV episode (most likely) or a movie that I watched as a kid, probably sometime between 2005–2015. It was aimed at children or young teens, not a horror.

Here's everything I remember (some details may be slightly wrong because it's been so long):

A group of kids becomes curious about a mysterious middle-aged woman who lives alone next door (or nearby). Nobody seems to know much about her, so they start investigating her past.

The story reveals flashbacks about two young schoolgirls (I think they wore school uniforms).

The girls sneak into a restricted area near the sea/ocean with metal railings and industrial-looking machinery (maybe a dock, harbor, sea wall, pumping station, or something similar).

One girl accidentally causes the other (or she did it by herself) to fall. The second girl ends up hanging over the water, and the first girl tries to hold her by the hand but can't keep holding on. She eventually loses her grip, runs for help, and the other girl dies.

Throughout the episode, the audience is led to believe that one specific girl survived and grew up to become the mysterious woman.

The big twist at the end is that everyone (including the woman herself, I think) was wrong. The flashback is shown again, but this time the girls' faces are effectively "swapped," revealing that the woman was actually the girl who died, not the one who survived.

After realizing the truth, the woman leaves somehow. I don't clearly remember whether she literally disappeared or just left peacefully.

I'm not 100% sure about every detail, but I'm very confident about:

• the mysterious female neighbor,

• the two schoolgirls,

• the industrial area by the sea,

• one girl hanging from the edge while the other holds her hand,

• and the final reveal where the flashback is replayed with the girls' identities reversed.

I think it wasn't a well-known series,

This is "Whatever Happened to Sarah Jane?" (2007), S01E07-S01E08 of The Sarah Jane Adventures (2007-2011).

It's a spin-off of the more well-known series, Doctor Who, focusing on the exploits of Sarah Jane Smith, formerly a companion of the third and fourth incarnations of the Doctor in the 1970s, but now tackling cosmic/alien threats independent of him.

In this two-part story, it's revealed that when Sarah Jane was thirteen years old in 1964, she and her best friend, Andrea Yates, snuck onto a pier that was closed off to the public (in their school uniforms), and Andrea ended up falling over the edge and clinging on for several seconds, before falling to her death.

However, an entity called the Trickster went back in time and offered Andrea the opportunity to swap places with Sarah Jane as she was clinging to the edge of the pier. Andrea accepted his offer, and as a result, Sarah Jane fell instead of her, and an adult version of Andrea now lives in Sarah Jane's house in the present.

The only human who remembers that Sarah Jane is supposed to be living in that house and not Andrea is Maria Jackson, a girl who lives across the street. Maria goes to the house to visit Sarah Jane and is mystified when she finds Andrea (a woman she's completely unfamiliar with) living there instead.

Maria investigates Andrea's past and eventually manages to persuade her to revoke the bargain she made with the Trickster. In 1964, the teenage versions of Sarah Jane and Andrea swap places on the pier again, with Andrea falling to her death, and the adult Sarah Jane replaces Andrea in the present.