Good point Super, to be honest I don't really know. Although I suspect like most train sets the kiddies won't be playing with it alone anyway and there will be bigger hands to help getting the trains on the track. The use of magnetic couplings though tend to confirm they are being made for easy use so I would suspect they've gone for the best option for kiddies. The other thing about small wheels is you can limit the speed and maybe remove the need for any level of complicated gearing thus making the motorised models more reliable...
Happily collecting things all my life...