The tag-a-long is probably your best bet!
If she really doesn't want to ride,
Wike has some interesting trailer options, including a kid trailer that's spec'd for up to 100 lbs. and a 'special needs' trailer that holds a rider up to 125 lbs./64".
My hauling needs have been accomplished quite well with a 1990s fully rigid MTB running slick tires. $100 for the bike, $60-80 for appropriate tires, plus whatever for the trailer ends up being significantly less expensive than a cargo bike.