I was in the LBS last week and saw a 16" Early Rider Belter.
Early Rider are claiming 5.6kg and using the ol' 1-hand-test the weight is night and day compared with the 16" Hotrock that was sitting next to it. It's a very nicely put together wee bike and certainly unique with its belt drive.

Also the Islabikes Cnoc 16 (which already put most of the competition in the shade) has gone up in price for 2013 but also gone on a serious diet and is now a hair under 6kg.
So we've now got some great 16" options available in the UK for 2013.

On another (related to kids bikes) topic, Chain Reaction Cycles has 20" Schwalbe Mow Joe tires on super special (57% off) at the moment if anyone's in the market. I took the liberty of ordering two of the 2.0" for when I'll need them, probably around this time next year .