Character mistake: The Sheffields are going to Miami but when they are there, Maxwell mentions having gone to Disney World on the same trip. Disney World is not in Miami, it's in Orlando.
Continuity mistake: When Fran's in the kitchen making an ice cream sundae she tops it with a lot of whip cream, but after Fran pulls a tissue out of Gracie's shirt, when she and Maxwell sit down all of the whip cream suddenly vanishes, but then it reappears.
Answer: Mr. Sheffield needed someone to manage the kids, not necessarily babysit them. Brighton was a Bart Simpson type, Maggie was shy and awkward and Gracie had personality issues.