And then Elmo happened and then Barney happened, and things have been on kind of a downhill slide into brightly colored, C-major MIDI music Cocomelon happyland.
Classics that sound great sung by Raffi, Ella Jenkins and even Caspar Babypants are some sort of ear mush when done by Cocomelon, Kidz Bop et al.
Compare with Pokémon Kids TV, which sounds really cheerful, and you can hear the singer is really enjoying themselves. Also mostly the same classic children songs, but so much more enjoyable to listen to.
The generic midi versus funny experimental instrumentation play a part too.
Where many people go wrong is that they assume little kids are stupid instead of understanding that they lack context.
Kids are literally just the definition of inexperienced.