I recently had a frustrating exchange a store (that shall remain nameless).
Me: "I would like to exchange this shirt for another size"
Clerk: "Do you have a receipt?"
Me: "No, I got it as a gift."
Clerk: "I can't do the return without the receipt."
Me: (more frustrated) "I just want a size that fits me."
Clerk: "I am sorry, I can't do it without a receipt."
Me: "Let me talk with your manager."
Clerk: "She will tell you the same thing."
I ended up being able to exchange the shirt after some further frustrating interactions.
The lesson here is that our interactions with our kids are at times like this. We try to convince them to do something they don't want to do and they resist.
Your job is to remain calm and put in the actions that will increase the likelihood they will do it. No guarantees, but as the parent your job is to not get side tracked into the argument and remain focused on the goal---for your child to complete what you asked them to do. Your child doing the task with a positive attitude is optional (so long as it is not abusive). The bottom line is completing the task.
Avoid the argument. Avoid justifying. Avoid over explaining. Stay short and to the point. Have your backup plan if they refuse. Those are the keys to getting things done.