You've little hope of blagging a free upgrade with kids to business class because of a fractured leg, infact, as the injury is so recent there's a risk they may even ask for a Doctor's report to say she's fit to fly (depending on where in the US the flight originates) see:
7.3 here if you were to make too much of a fuss of it at check-in. . .
100% convinced if she turns up with her leg in plaster and 2 kids in tow they will bump her off the flight.