When someone pays for all someone else's expenditures (including food), for example a child, what do you call it? I want to construct, if possible, something based on this structure: 'someone is one someone else's [sustenance? maintenance?]'. I also want it applicable to the situation when one state provides for another country (due to political reasons, for example) to the point when the latter country is not viable on its own.
The child (or the country) receiving support is called a dependant (Br.E.) / dependent (Am.E.):
1 : one that is dependent
especially : a person who relies on another for support
// an individual's spouse and dependent
Guardian; noun 2. A person who is legally responsible for the care and management of the person or property of one who is considered by law to be incompetent to manage his own affairs, as a child who is a minor. (source: American Heritage Dictionary)
Also try Custodian, Trustee, Superintendent, Warden.