I've no experience, except that I've never seen either kind of seat on a Brompton. I guess it depends on the age of the child, but I saw someone locally with what looked like an extra saddle that was attached to the main tube, so the child was sitting in front of the rider, but behind the handlebars. It looked fairly raised, and I can't remember if there were foot-rests.
I didn't get a great look at it, and the child was probably about 5 years old and seemed pretty happy. It looked like a good solution that didn't look like it affected folding.