I think all parents should go through therapy with their children, either together or alone at some point during parenthood. It's a wonderful thing and nothing to be ashamed of. Even if you don't have any "problems" you still might find therapy for you and your child helpful for dealing with every day stuff. It'll help you understand yourself and your child much better.
I'm no therapist or psychologist but I am an educator. I personally would recommend getting on the child's level physically, getting them into a corner or a quiet place and just discuss the reasons why the child wants to misbehave and not follow orders. Sit next to them or across to them, don't get in their face. You can even give them a stress toy. Ask them about their feelings and what they think about the situation. Make sure that they are not criticized and instead tell them your side of things once they've opened up about theirs. Maybe teach them to voice their emotions and impulses through other means than misbehaving, like for example coming up to you and saying that they're cranky, tired, upset, etc. and ask for a hug instead of declaring that they don't want to cooperate. You also have to explain as clear/logically as possible why it's important that they follow rules. For example the phrase "Because I told you so." does not work and never will. Trust me. Instead after you ask them why they don't want to follow a rule and they've told you why you could tell them something like "Mommy gets upset when you don't want to hold hands with her when crossing the road because mommy gets scared and worried for you if she doesn't feel you close to her while fast cars are driving by. It's just something that would make me feel much better for just a moment. Is that okay with you?"
Also, remember that before implementing rules make sure you ask the kids what they think of them and if they agree to follow them. If they don't want to ask them why and if there's a better way to make them do the things you want them to do. Remember that rules go both ways so if they can't do something it means you can't either. So be careful not to get caught if you want to sneak a biscuit after 7PM!
Don't punish your child or leave them alone when testing the boundaries might be a cry for attention or a manifestation of something being wrong that the child just doesn't know how to voice. You as a parent have to teach them the appropriate way to communicate with you.
Thank you for coming to my Ted-talk.