lightweight children bike 12" ?

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I was looking at children bikes, and very disappointed. Children bikes are very heavy. E.g. the manufacturer scott (calling themselves "the carbon experts") has a 12" wheel bike, that weighs 8.5kg - that's more than my bike!

Does anyone know of lighter commercial children bikes (age 3-6), of home/custom built bikes, or even where to find componentry to build a childrens bike?