Roccobike had the right idea here. People just don't want to pay that much money for a bike that their kids are going to outgrow in a year or two, and therefore spec cheaper and heavier components. If you're really that obsessed with building up a super-light 20", your best bet is a race BMX.
This complete should be pretty light:
http://www.abikestore.com/Merchant2/...Store_Code=abs
And it's also available as a frame only, so you could probably build it up as a custom and make it even lighter. Expect to pay for that though...building up BMX bikes isn't cheap.