Staying calm when your child is misbehaving can be a difficult thing to do. I think part of the answer lies in where you place your focus. Often parents feel responsible for their child’s behavior. The misbehavior feels like a direct reflection on you. If you think other’s are judging your parenting based on the child’s behaviors, it is easy to lose your cool. To calm, shift your focus. Think of being responsible to them rather than for them.
For example, you and your three-year-old are having lunch together at a restaurant. They are busy eating their mac & cheese when all of a sudden, they fling a forkful and hit a person at the next table. You are not responsible for them throwing food. You didn’t arm and aim them. You are responsible to model, teach apologies, to address and curb the behavior. You are responsible to teach them how to behave in restaurants moving forward.
Another example, your eight-year-old decides to skip spelling homework and studying for a week and gets a poor grade. You are not responsible for the grade. It is not your homework. You are responsible to help them understand the importance of homework and studying moving forward. You are responsible to check their homework completion in the next few weeks while they get back on track. You are responsible to sort out whether this was truely a dip in effort or a bigger learning difficulty.
Overall, this means to focus your efforts on what you can control. If you are so narrowly focused on changing their beahavior, you are likely to feel frustrated. Focus on the piece you can control. Rather than focus on changing their behavior, focus on changing your reaction to their behavior. Focus on building skills to better address, manage and teach about behavior.
Parenting by Dr. Rene 
What if my 7 years old understands very well why he goes to school and why he has to do his homework and still argues about it and prefers to skip it????
Many 7 year olds would rather stay home and play or lounge after school because it is the easy and fun option. There are a few 37 year olds I know if given the option of work or a day off at the beach, would take the beach every time. It is also difficult because the long term benefits of good grades, meaning better future options such as class choice and college acceptance rates are lost on a 7 year old. I think the general answers are to give a lot of empathy when he pushes to not go help him to manage the homework when he prefers to skip it. This means breaking it into smaller pieces, providing the study area and time, and making what you can more interactive or playful.
Other helpful responses here?