I don't know of any kid specific irons, but an Antex 12W or 15W iron has a handle diameter smaller than my little finger. The slightly wider "guard" part is about (only slightly more than) the diameter of a tube of lip balm. It really deserves to be called a pencil iron. Not like those jumbo hot dog size irons. I can't use those things.
Oh, you will want to "work" the power cord a bit to soften it, or the stiffness will flip the tiny iron around. They do make a soft, silicone power cord, but I've only found them on 240V european models.