Yes, your understanding is correct for your the first question. For the second, it's not clear what you mean by a non-US resident alien. Any US person- citizen, permanent resident alien (whether living in the US or not), or US tax resident (by substantial presence or election) must file the FinCEN 114 and/or 8398 as applicable.
The dollar amounts that trigger the Form 8398 filing are different for residents of the US versus US persons who live outside of the US, but other criteria remain the same. (See more about the dollar amounts HERE.)
The US citizen child files when he or she meets the criteria for filing a tax return. This criterion is the same for children living within the US and those living abroad.
A child can be required to file a tax return even when a dependent of their parent, particularly if the child has significant amounts of unearned income (from dividends, investments, or capital gains) or has income earned from work or self-employment. The IRS has a wizard that helps determine when an income tax return must be filed Do I Need to File a Tax Return?
A US citizen child with foreign assets in bank or investment accounts that exceed $10,000 on any day of the year would also have a filing requirement for FinCEN 114 (FBAR).
While those issues aren't especially common, it is important to be aware that there are instances in which a dependent child living abroad could be required to file a tax return.
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