Even aggressive responders should be able to physically overpower a 9 year old and get her in the car without pepper spray.
I watched the full video and I am more confused than before. They already had her handcuffed and halfway in the car. Even if I buy the explanation that it's haaard for a literal army of physically fit officers to get a thrashing and kicking 9 year old into the back of a car and shut the door, she wasn't doing that. She was just sitting there. There appears to be no reason whatsoever that they could not get her the rest of the way into the car and wrap this thing up. They have her butt on the seat for several minutes giving her contradictory commands (Sit up. Get in the car. Stand up.(?) Sit back.) until they apparently get bored. Then they calmly ready their pepper spray canisters and spray her in the face.
This is not another case of cops going in too aggressively and acting before taking even half a second to assess the scene. They took absolutely all the time in the world to assess the scene before deciding to pepper spray a kid in the face because at that point they apparently just felt like it.