This is the bike (image ripped from an ad, not that bike but the exact same model):
http://imgs.advertentiex.nl/upload/Te-koop-prachtige-christi...
So you can see that in the back there are no options other than to either completely change the frame or to go for a single speed and neither is feasible. I could maybe expand the space between the forks in the rear so a wider hub with a motor + cassette would fit but that risks cracking the aluminum frame.
The bike has a rear hub that is geared, so there is no space there and this being a cargo bike means that an after market mid-motor can't be placed (it would run right through the middle of the main spar. That leaves the front wheels for the electric drivetrain. I'm not afraid of slipping or sticking, electric motors tend to be pretty well behaved in situations like this, the difference in speed between inner and outer wheel in a turn will be readily absorbed by the motors and discarded as heat, and they won't be overpowered to the point that they will start turning independently, each motor will run at a leisurely 125 Watts or so. The one situation where that might happen is if one wheel comes off the ground but in normal use that should never happen.
This is the kind of kit that I'm eyeing to do this conversion with:
https://www.fietsunie.nl/Babboe-Big-bakfiets-elektrisch-make...
This is a pretty common thing here in NL and rather than build the whole drive train from parts I'd go for a kit.