in your Pedal car fabrication look at how adult upright trikes are made
a half width axle + a fixed cog driving 1 wheel would be connected by a a short chain to a coaster brake hub with a second sprocket attached to the hubshell .
the cranks have a separate chain turning the driver cog . You need chain tensioning movement for both chains ..
Power and braking are just applied to one of those 4 wheels , so I would not be ambitious about speed .
My problem is though, that I don't have anything that the brake arm can secure on to
build a surrounding bar to support both ends of a regular bike wheel?
who is helping the metal work on this project. ?