What I would do is to sell the Kona's and get a couple of Yuba's with the mid drive kit from here:
http://www.urbancommuterstore.com/ The key factor is that this system drives the bikes drive train which is a big advantage and why the other poster recommends the Panasonic as it does basically the same thing. Much more reliable for carrying loads.
Looks like a Yuba with that system would be about $2300 or so and then you would need a battery. Don't skimp there, figure out how much you will want to be able to travel during the day and get one that will get you that far. Plan on $500 or so for a good one. So for under $6k you would have two brand new machines that will fit your needs very well. Use the money you get from selling the Kona's to get an electric trike going for the boy that has a pedelec system on it, so that it will only work when he pedals, that he can rip around the neighborhood on and you will still be money ahead of your $7k goal.
If you feel challenged by all this then I would go see Wake at the ebike store in Portland,
http://ebikestore.com/2-ebikes. He shows a different motor setup on a Yuba on his site but as that is no longer available I am sure he would be willing to work with the mid drive. He is a Yuba dealer also which is a plus. BTW the Yuba has far better carrying capacity than the Kona, disc brakes and is all around a more sturdy bike.