Is it doable to add gears to a single speed bike?

My eldest son is asking for a bike with gears. I can't seem to find a suitable one. Just found out that Dahon curve is still too big, the distance between the saddle and handlebar is too far. I am thinking of adding gears onto the monotine single speed foldie he is now using but I think the rear derailuer may strike the ground when fully extended in lowest gear as the bike is on 14" wheels...

not recommended because it would cost a lot, since a lot of parts need to be changed. I believe there are kids' bikes with gears, but I'm not sure where we can find them in Singapore. perhaps someone else will know where.

http://www.polygonbikes.com/index.php?pgid=d_bike&bikeid=278&par=8

I duno if your son can fit.. can go down rodalink to try. but recommended to call first before going down as some branches may not have.

contacts here: http://sg.rodalink.com/outlet-location/

Wow... full sus kid bike