Oh man, that brings back memories.
Many years ago, when powerchairs had belt drive, I built a trailer, similar to this, for one of my customers to take his little boys(2 & 5)shopping with him. One in the trailer, the other on his lap. Several months later, he came in with both rear axles bent. This is what he told me:
He had taken the 2 boys with him to get some stuff at the hardware store. he loaded the trailer with 3 bags of cement, and some cinder blocks, and put the one son on his lap-and the other rode his bicycle.
On the way home, the bike had a flat tire-so it went on top of the cement. and the son stood on the back of the chair, holding on to the push handles, and they drove home-on a bumpy sidewalk......by the time I saw it, the rear wheels had about a 20 degree angle on them