I pass an elementary school and see a few regulars during the nights parts of the year. I think the simplest would be the '"half rear bike" that attaches by bar to the main bike making it a tandem of sorts - and allows the child to pedal along. They seem less bulky and cumbersome than trailer and lower center of gravity than a seat.

The funniest thing is seeing the girl slumped over sleeping while her Dad's biking