Well, there's more dark material to just Littlefoot's mother death. There's also the repeated threat of the Sharptooth and who knows what other threats.
The Brave Little Toaster had some other "dark" scenes. The blow-out of the Air Conditioner was a pretty darn SCARY scene...and he was a scary character to begin with. Then there was the lonely flower that wilted when Toaster ran away from it, taking its reflection away with him. The nightmare with that...clown (holy cow, that scares ME), Lampy electrocuting himself (a scene that would be very upsetting to kids warming up to him), and the entire junk yard scene in which EIGHT cars get scrapped...in song. And let's not forget the magnet, who repeatedly tries to make sure that everyone gets smashed, resulting in the Master getting trapped and nearly killed himself until Toaster jumps into the gears. Oh yes...and the junk shop scene, when the blender gets taken a part. And the Peter Lorre hanging lamp might be pretty creepy (NO! Don't use that bulb, Lampy! Who knows what kind of B-movie germs could be on it!)
I guess the BLT feels a little darker to me because there are so many scenes that are so much SCARIER (The air conditioner, Toaster's nightmare, the parts shop, the junk yard, the magnet, Toaster AND Lampy appearing to die, the Master just BARELY suffering a very nasty death, the storm scene...)
In one of the sequels, Radio is dead for quite some time because he sacrifices his cathode tube.
