Trek has dual hole square taper cranks on a bunch of their kids bikes including the Gary Fisher kids bikes, and maybe more. They have both 152/127, and 140/120. I emailed them to find out about getting the cranks separate from the bike and they said they don’t sell it as an after market item.
The next step would be to go to a Trek dealer and ask them to call for Trek replacement parts, and they may be able to get some that way. The ones I’ve seen have steel chainwheels permanently attached to the right side crank. That would be easy to grind off, or use a hack saw to cut the spider. They are the same cranks Lobbybopster used on his “road machine” in this thread.
I came up with a plan for drilling holes in a pair of cranks to make them dual hole, but have yet to do it. I do have the cranks now. At this point I’m just waiting to be able to afford the tools I’ll need to do it. There is a thread where I was discussing the idea.