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some Copenhagen kids are lucky if they get an innertube to cling onto, the true cargo solution is just to have them cling onto the enormous box/fridge/sofa (delete as appropriate) and hope for the best
in all seriousness I've had the pleasure of riding as a passenger on the Omnium Taxi-Bike and as an adult it was quite comfortable so I'm sure the Omnium Kids seat mount or even the lesser known 'Omnium Backrest' would be very comfy for a child.
That is such an unprofessional answer.
Show him how to strap two kids on the top rack with just a couple of innertubes :-)