She has fairly severe dementia with both short and long term loss of memory, but one day the tv was on for a very old Lassie movie, shot in the UK probably in the 30's or 40's.... and she was able to follow the story and enjoy it. I'd love to discover more movies she can enjoy.
There are 2 movies in a series which I can't recall right now; they deal with efforts to rehabilitate marine animals, including a dolphin which lost its tale. They're generally light hearted but there are some "iffy" scenes when it's unclear if the dolphin can use the artificial tale designed for her.
I don't know if these brief scenes would affect the overall tone of the movie for someone with dementia.
If you have cable, Hallmark movies, especially those at holiday times, are often bonding and light hearted ones. They're the kind that leave you with a warm and fuzzy feeling after the movie is over.
Fly Away Home is one of my favorites. It's the story of a young girl whose mother was killed in an accident; she moves in with her father, has some adjustment problems, but finds solace in caring for orphaned Canadian geese, eventually leading the geese south for the winter in a home built glider type device built by her father. The last scenes are real heart string tuggers, and close the movie in a very, very positive tone.
Elizabeth Taylor made some animal movies, although I can't remember the names. Black Beauty was one.
There's an interesting series titled Adventures of the Wilderness Family, about parents who move with their family to homesteading in the wilderness. There are a few unsettling moments, but overall, it's lighthearted and heartwarming.
I suspect there are categories of movies to rent if you have cable, or if you have a VCR or DVD, check out the older movies such as the Lassie ones.
Cracker Barrel has a fairly decent collection of older movies. My father gets his old John Wayne and WWII movies from a magazine selection; it might be Traditions.
You can research online to find family oriented movies to buy.