I'm not sure how the coolers/freezers maintain temperature on those bikes, but I think the primary goal of any batteries on them would be to keep the ice cream from melting.
I am wondering how effective a 250W wattage gear motor on those might be if powered by a 36v or 24v solar cell canopy!! Shade for the rider/employee = no sun burn = no heat exhaustion. And just enough power to run the bike without going too fast for the neighborhood kids.
http://www.frankentrikes.com/Specifications.html
And you could still pedal it fast enough to get away from the kids with bats. ( A friend of mine and her youngling friends actually did this to a Good Humor driver to see how he would react. My friends are great!)
EDIT: A quick search finds this 22Lb, $800 100W solar panel,
Sunsei Solar Panel se 6000. If I am dividing correctly, a 100W panel, via and voltage converter could provide 2.5amps to 4amps at 36v and 24v respectively.