Was looking at those rock riders. Just out of interest has anyone cycled with a kiddie trailor on the back! Is it noticeably tougher than normal cycling?
When my kids were proper tiny, the missus and I used to cycle, with them on kids seats on the back....I had the older one, she the younger....worked well for maybe what seemed like a couple of years, but could have been less, then they get too big, but not really big enough for proper cycling.....it was a good few years before we could all go out as a group.
As regards the seat and I guess it's a bit similar with a trailer, it's harder going, but the likelihood is you'll likely be doing it because you're a cyclist, and used to a bit of suffering.