"It's Made For Kids" is an excuse used to dismiss criticism towards TV/Web Shows made for younger audiences.
There is also an article on this topic on the Crappy Games Wiki.
It is well known that kids are easier to entertain, even by simple things that adults may find boring. Likewise, they are unlikely to understand and/or appreciate more highly complicated and mature adult shows. Because of that, shows targeted at younger audiences tend to be simpler than shows targeted at older audience, but that doesn't mean that shows aimed at kids should be too simplistic and basic.
Often when shows meant for younger audiences are criticized for poor quality, relying on cheap comedy, using lazy animation, or lacking any real substance, the writers might claim that the show is made for kids, as an excuse for its poor quality. Saying that a show is for kids shouldn't be an excuse for the show to be of low-quality or slapdash. Show writers who rely on this excuse in order not to put effort in their work also treat kids like unintelligent beings with no taste.
Plus, if a show primarly aimed at kids is made in a certain way, it can even be good for or interest adults as well. After all, the TV parental guidelines rating TV-G means General Audiences, not just kids, and there are many examples of kids shows that are also enjoyed by adults. In conclusion, there is no excuse for making a bad quality show just because of who it's targeted towards.