w/apols for x-posting from the folding bike forum...

I'm considering buying a Mu XL for my other half as a birthday present. Her current bike is a horrible heavy German thing (a Fahrrad S200) which is ill suited to London commuting and in particular the hills round Hampstead. But I'd guess that some of the time she'd quite like to be able to take her bike out with the kids, one of whom needs to hitch rides in a baby seat. Has anyone tried fixing a childseat to a 20" wheeled bike? Is it even feasible? And if so, any makes of seat which might be particularly suitable? Any experiences of trying it and failing?