I threw together a trailer to carry my kids' bikes so we can occasionally ride our bikes to or home from day camp or after-school camp. The older one is ready for a 24" bike so I may have to modify the trailer a bit.

I took a generic toddler trailer, removed the canvas, flipped the frame supports front to back, and added a plywood floor,

20" kids bikes. The fork rests on the crossbar.

I use wire twist straps to keep the bike against the trailer frame.

I bolted a crate to the floor to carry the helmets.