You probably need a 52T front gear with an 11T small gear on the rear freewheel to get a comfortable cadence for 20 mph on a 20" bike. These are about the limits of what is commonly available too,

The only maker I know of that sells 11T freewheels is DNP. I put them on my 20" electric folders. One came with a 52T chain ring and 20 mph is attainable with the pedals. The other has a 46T chain ring and it maxes out around 18 mph for comfy pedal cadence.