For my mom I use an 80, though, to get a larger leg with regular sock yarns. And for my daughter I use a 54 cranked to tightest tension to make them small enough (she's 7).
I guess if I had to have just one, I'd use a 60 and not use commercial sock yarns (the original plan _was_ to make socks with my handspun, ROTFLOL)
(question came up on sockknittingmachines, 19April2007)