This is what I'd get for my kids back in Canada, if I had the garage for it... Nine feet long.
This has been well reviewed. It ain't perfect, and a Cetma is better, but you pay the difference (the Dutch brands cost more because of duty and shipping - poor value). It is an outstanding deal for this kind of bike: $2000 US to $2500 with some options. These guys should get as much press as possible. There are competitors out there, but none that don't cost $1000 more, or much more. Some Chinese made pieces of shit will cost half, and last a fraction as long, handle and stop poorly. How much are your kids worth?
However, as the mechanic in the third link points out, box bike are looong and noodley. Something like a Bilenky cargo is much less trouble, though you need more skill for loading the kids and getting underway. Of course, the Bilenky is something like $6K; the more reasonable Circe Morpheus $3K, and shipping from the UK; and something the same for this; the $2K Kidz Tandem is a bit graceless, and too long; but Soma is going to make up the 'Pick Up Artist', a shorter version, for $1.5K, which is a steal!
Yes, a trailer and options costs only about $500, but with this you can see the kids and reach out to bop them on the helmet when they need it.