As a instructor I have the benefit of working with 100's of kids in every age group. As well as being part of larger networks of experts and other instructors.
So I want to share some useful tricks me and my team use in class to work with different sorts of behaviour.
The thing to remember is that often kids get stuck in a specific mindset, and in order to get past it you have to change the approach.
For example, if we have a child that doesn't want to do a technique pushing them to do it when they have decided not too is unlikely to work.
Defiance is part of kids finding their own way, learning to develop opinions and preferences. Once they have decided "no" changing that stated opinion is tricky.
One option is to give them a choice, rather then trying to force them to go against what they have already expressed. Even once they decide they do want to do it, they will have a hard time contradicting the opinion they already decided on.
So instead of and order "go do the technique" it becomes "do you want to do the technique with Sarah or Paris" and often that is enough to give them the sense of choice they desire.
This same tactic can be transferred to other aspects of their life as well. Just remember that often defiance is simply a desire for choice.
"Do you want Mom or dad to tuck you in?"
"Do you want to wear your red shoes or blue shoes today?"
"It's time to go, what song do you want to play in the car?"
Offering a choice gives them some control and allows them to express preference. And once they have expresses it, just like once they have expresses defiance it affects their mindset. Once they have mentally and verbally committed to the red shoes, they are far less likely to refuse to put shoes on at all.
Give it a try and let me know how it goes!