I would hesitate to even call "The Mouse And His Child" a "children's movie." It is, like "Charlotte's Web," fixated on issues dealing with death, or at least the loss of things...loss of home, loss of family, life without a support system of friends, relatives or financial security...the film even the questions how two individuals who are connected can be so far "apart" ultimately. "Lady And The Tramp" this is not.
The Peter Ustinov-voice "Manny The Rat" character who exploits broken and abandoned toys for personal gain is cold, greedy and mercilessly cruel; one disturbing sequence that few children who see the film can ever forget finds him forcing a sad, broken elephant toy into "early retirement;" after the pathetic creature begs for its life we see it hacked up for firewood in the shadows on the walls of a cave while the other horrified toys look on. Even Bambi's mother was killed off-screen, and followed up with a dignified response by Bambi's father!
The parade of odd, and often screeching characters gets annoying after awhile...Cloris Leechman is particularly shrill in an endless segment dealing with an outdoor theatrical production, Andy Devine plays a another pathetic and slightly annoying frog character. Our "heroes" are passive victims of their fate and their inability to do anything to help themselves creates a feeling of frustration and despair in the viewer.
The quest to find "home" is fraught with disasters so unpleasant it seems impossible our friendly, soft-spoken, well-meaning mice friends will even survive, let alone end up happily...at one point the mouse and child are sunk into the dirty bed of an abandoned river and face a life of rusting away and being eaten by a fish as they sit staring off into space contemplating their sad existences. Even Charlie Brown won ONCE in awhile!
Finally the film is loaded with esoteric comments and asides, such as a running motif about a painting within a painting on a dog food can: "Can you see anything beyond the last visible dog?" or the deeper-than-dirt concept, "The relation of self to mud is basic to any discussion of to be."
The constant philosophizing of the flighty characters mixed with an unusually cruel and arbitrary villain and a quest that seems hopeless at best, tedious at worst, makes "The Mouse And His Child" a challenging film for adults and kids alike, and it's no surprise to me that it didn't take off and hasn't resurfaced over the years.