I don’t think you can lump Endeavour and Garuda together. Yes, they’re both based on Arch but Endeavours basically is Arch with a GUI installer and sane defaults while Garuda changes a ton of things and adds a ton of customisations that make it very different from a plain Arch (or Endeavour) system.
The UFP has a currency - Federation Credits. The average citizen may not need (or even have) money, but it exists and is presumably accessible in some way if you have genuine need. For example the station personnel on DS9 never had issues paying at Quark’s, so one can assume that they did get a salary because they were posted to a capitalist environment. I’d assume that child support would work on a similar “as needed” basis.