Bonsoir,
Merci pour toutes ces informations. Actually, I am extremely concerned by safety. As you understood, it is not just about what is possible : I just saw a child seat attached with zip-ties !
I also saw child seats attached to seat posts on folding bikes, instead of the seat tube. While I only weigh 60kg, I would still be worried about the strength of aluminum seat posts designed to be as light as possible. Hence also why I'd be interested in a Pacific Cycles Reach which is a foldable bike with a more standard geometry (i.e. with a "normal" seat tube). However, the child seats I'm considering mention wheel size requirements «Vélos 26 et 28 pouces» ! ☹
Thanks for the add-bike link. I might use this for my vacations at my holidays home where bikes outnumber cars, but would not feel safe enough to use this in Paris ☹.