Cube's new kids bike looks interesting -- hardtail 18" singlespeed at 7.5kg (~16.5lbs) with a list price of around $350.

Lowest shipping that I've been able to see so far from UK is ~$200, but CRC has some of Cube's other line and may have this once it's released.

Has an interesting chain guard design that I'd have to imagine you could lose quite a bit of weight by removing.

Cranks are 114mm w/ a 32T chainring. Specs aren't clear, but it's likely the same square taper BB used in their 16".

Cube Bikes launch new 18" wheel kids mountain bike - Cycle Sprog