is it possible to update fields via workflow (without SuiteScript) on existing record based on fields from another record type (let's call it source record)? Both records are non transaction type and they have parent/child relation. Moreover that needs to be done at a specific daytime which is written on a source (child) record field.

Yes. The happy path is to just make this a scheduled workflow and your saved search will contain all the criteria of X field on parent, Y field on child [are different or whatever applies]... re: time of day. Options: • you could run this more frequently than 1x/day in workflow and indeed you may have to based on the number of records you are handling • Use the criteria of the saved search to control when records will be processed if it is possible... • schedule a mass update using your workflow