How do you take the freewheel apart on the back of a 20in bicycle wheel?

They're threaded in place. You need a special freewheel puller (park tool sells them, among others.) to remove one and it can be torqued beyond belief. They spin on by pedalling action, so no need to torque that one.

There's a newer freewheel design that sort of mimics the design of MTBs, with a freehub body and a separate sprocket. they need an different approach.