As atrocious as it looks, I don't think the long pants are solely to be blamed. Upon further inspection, I don't think she tripped on her pants, I think she just tripped over her, what looks like, wedged open-toed espradilles. If I know shoes, I would say these are one of the hardest to walk in. Way harder than walking in 4-inch stilettos! I have only two pairs of wedged shoes and I rarely wear them because whenever I do I risk falling on my face at least twice per wear (although I'm willing to risk a few embarassing trips because they still look good). If anything, Britney was stupid enough to juggle a cup and a baby whilst wearing wedged shoes. These shoes should only be worn, with nothing in your hands, and maybe a shield, because you'll need to break your fall!
And while Britney might not be the best mother (I bet she feeds the kid Cheetos for breakfast), there are far worse parents out there, who don't wear wedged shoes, that need to be put under similar scrutiny.