Is 20,000 RMB enough?

Depends my job in Shenzhen and am having trouble finding a job in Shenzhen. It depends on how you want to live. Sounds like you can have a good opportunity. It seems like you have a good chance to live. If you can be happy, take it.

It's not nearly enough to support a family of four in those places you have listed.  I would classify that as near poverty.  Teaching doesn't pay much but for a principal at an IB school you should be aiming for a lot more.

If you add in your part time salary of $4k a month then it's not as bad but it's still not going to give you a great quality of life.

Is there a reason why you can't just find a full time job with low hours in China and do your business on a full time basis?

Principal in an IB school? 20K ? That is incredibly low. I am just a teacher and earning almost 50% more than you as a principal. Think about it again.

Hello, i'm Luis from Spain.My english is very poor, sorry.
Maybe this link can help you.
It is only indicative.  You can also check the prices of other cities in China, you may even be able to find your destination city.

https://www.expatistan.com/cost-of-living/tianjin

That's horrible pay as  a principal.   You should be earning in the range of 35-40k a month at an IB school.  I'm a teacher and I bring in 23.5/m before taxes.  You shouldn't have to touch your part-time income to support your family here, period.

Way too low for a principal working in an accredited IB school.  No stipends for the principal role and responsibilities attached to it?

It's a low salary for a principal but it depends on your qualifications. 20,000 RMB after tax is more than enough to live on comfortably. You're not paying rent, food is cheap, utilities are cheap. Even with 4 kids, that's still enough to live off comfortably. Maybe it's not enough for the twentysomething tourists that come to China to 'teach English' and spend their free time getting drunk, but for responsible adults, it will provide you with a comfortable standard of living. Food costs for families are between 4000-5000 RMB in Beijing, let alone those locations. I can't see water and electricity being more than 600 RMB a month. High-speed broadband is about 100 RMB a month. 5G with unlimited data is about 120 RMB per phone. That's 6420 RMB so far. Then you have travel costs, clothes for the kids and going out. As long as you're not buying designer gear and flying out of the city every weekend, you're going to have plenty leftover from 20,000 RMB.

Hi, I am teaching in an IB school in Changsha and earn as a normal teacher more than 20k (also school pays taxes, apartment, etc.). Principal for less than 30k-35k can't be ok...