You sure you want to skip all the way up to 20" wheels?
That's a pretty big step from a scoot-bike (I'm also assuming your kid is around 5 y/o).

I have a nice 20" Redline BMX mini that my son raced when he was 5/6, but it's handbrake only and might be a little much for a kid just learning to pedal, though once he's got a bit of experience, it's a super sweet first "real' bike. Fits smaller than your standard 20" bike, only weighs ~15lbs.

Also have 16" Redline Pitboss that has both coaster and hand brakes (and flames!). Also would be a cool first bike.