The dog may kill them. I would just keep them in the pen. It looks like it is small enough to move around the yard.
Oh, I am near positive the dogs would kill them, that was mostly annoyed sarcasm on the fact that my dogs wouldn't let them be. :)
We do plan to make them a bigger pen, as the 'tractor' is insanely heavy (re-used free wood, doesn't necessarily mean light, lol), and our property we realized is much bumpier/hillier than we realized, when we went to place it, pretty much nowhere worked.
But no worries, our chickies hang out in the run of their coop lots now! :)