Dutch bike for adult carrying two children
Among the various types of bikes sold by the De Fietsfabriek shops in the Netherlands are bikes for carrying two children. I do see a fair number of instances of multiple children being carried on Burleys, but the children aren't strapped in, so it is unsafe.
The next stage of the take up of biking as transportation will be a greater variety of bikes being made and marketed to smaller, more specific segments of the market.
Still, the inventory presented in most US bike shops is weighted toward what we'd call "racing bikes," which sell for a lot more money than a low cost city-focused bike, which generally can be bought for $400 or less.
That makes sense, as the market for biking as transportation is still quite small, focused on major cities.
Although we are seeing more examples of Dutch-type cargo bikes being used in the US, often for carrying children.
And younger children are accommodated within biking by carriers, add-on bike attachments, creating a kind of tandem bike, and trailers, along with their own bikes.
Image from the WalkBikeJersey blog.