I agree. The perfect kids' show shouldn't be just "Life's all lollipops and lickety-doos", nor should it be "The world is a cold, hard place to live in and then you die", but rather a medium between the 2 of them.
Let's give the young'uns some credit, shall we? They don't need to be talked down to. 'Sickeningly sweet' shows portray an unrealistic view of the world (aside from just not being very entertaining), and exposure to that and nothing else will turn your children into goody-two-shoed little wimps who will get walked all over as soon as they go out into the real world. But little kids don't need to 'keep it real' either; they should have some kind of positive reinforcement. A show that 'keeps it real', i.e., showing that the world is full of crime and pollution and corruption and evil and badness is just going to turn them into angry, bitter, depressed Goths. Ideally, a good show would combine elements of both: it would show that everything is not all sunshine and cotton candy, but at the same time, show them that it's not a bad thing to have moral values, strength of opinion and rugged individuality. Show them the harshness of life (crime, bullies, pollution, etc.), but also show them that it's not all bad and that there things you can do about it (the alternative to crime is justice, safety and prevention, bullies exist, but show them as the sad, pathetic individuals they are and don't glorify them by having them never receive any sort of punishment for their misdeeds like they do on so many animated shows unfortunately, and show the consequences of abusing the environment.)
But first and foremost, a show should be entertaining. Don't make it too preachy either way.