It occurs to me that you might do well with a simple seat on that rear rack, instead of a trailer. A couple with three young children in my circle have one on each of their bikes, and they go all over the place like that.
Regarding a single track trailer. Your wheels appear to be 20" which is typical for foldable bikes like that. My road bike wheels are 26-27" or so, and the single track connection clears it with ease. I’m sure, even with your rack, you’d have no trouble with a single track connection to the seat post. Main thing to think about would be when approaching a gradient change - assuming your seat tube is lower than mine - to go at it on an angle so the trailer doesn’t bottom out. I’ve taken mine on some light trails and have never had a problem though.
I don’t have any experience with hub drive bikes, but if you were to go with a chariot type trailer, the connection bracket could be installed on the spindle, might just need to make that smaller hole big enough to accommodate your axle.
Happy to help.
I should have mentioned a slightly different option of rack seating is basically to install a cushion, some rails, and running board to keep the little legs from flopping around.
The rails are kind of hard to search online for because they get called a lot of different things. I found this one by Surly, and this other one by Yuba.
That couple I mentioned before have a set of these on the one bike they take two of their children on. I don’t think they have running boards on their bike, but I’ve seen other people use them and think they’re a good idea. Here’s a post I found from some guy that made his own for a few bucks.
Benefit of the rails over the specific child seat is the rails still help hold cargo for when you need it.
Keep in mind, this whole kid on a rack option will change your centre of gravity, and starting, stopping, and slow maneuvers in general will feel a bit different.
Hope it works out!