I´m in the process of divorce and live in Mexico. I´m a USA citizen. As my son is 3 and a half years old, and my daughter is 19 almost 20, do I have to have medical insurance.
If you are a US citizen, living in the united states, then you have to have coverage and your dependents do too.
If however you spend more than 330 days in a row outside of the US, then you don't have to have coverage for any of those months (not necessarily the whole tax year).
Cost assistance is based on income and how you file. Make sure to get covered during open enrollment if you are living in the US, if you move back then you will have another opportunity to enroll due to "special enrollment".
Learn more about how the 330 day rule works.