No they couldn't, this is not Miami vice. The office was too close, he only had a downward angle to shoot, the body mass was between the gun and the suspects knee, he is trained to aim for the body mass, this is the same reason he didn't go for a head shot (not Call of Duty) or tried to shoot anything out of his hand (not Dirty Harry)
He should have either stopped him before he could open the door or moved back to reassess the situation and run the possible risk of having an armed assailant in front of him with a shield of children behind the now armed assailant (note when he went from suspect to assailant), the officer was pushed into panicking and reverted back to very basic training, aim at body mass and fire (7 was very OTT)
Just to be clear I am not trying to justify the police officers actions, but in this case it took 2 to tango and the were failings on both sides.