The only one i would feel safe for my kid would be this one because of two mount points for safety and stability:
https://www.google.com/search?q=brom...ml%3B480%3B344
I saw a few hooking all that strain directly on the seatpost with a child and seat probably weighing 30lbs+ total it just doesn't feel safe at all , and with that single point of failure that you and the child seat are putting weight on I would not bet my kids life on that since the torsional forces on that seatpost will get very high going over bumps and such due to leverage. For my kid if i'm going to take him for a ride it is on a non folding bike on a rigid child seat with multiple mount points for strength and stability. Some of these people are just gambling with both their lives if things go wrong.
That bikefun one looks nice but you are relying on a SINGLE bolt on the folding hinge to keep your child safe, a SINGLE point of failure on a part that was NEVER tested or even recommended for such stresses by the manufacturer. If that bolt snaps you and your child WILL be going over the bike and can get seriously injured going down headfirst. My folder I only use it for transporting myself but if i really had to I would attach a trailer to it like some here, that way if you go down it'll just be you.