I also have an extremely small head. I had to do some serious outfitting to get my helmet to fit (with the help of a ski/kayak shop in Kalispell, MT). I bought a grateful heads helmet with the thickest foam they had. Next, the ski shop had some extra padding for ski boots to make the liners fit. These were little sticking pad things. We stuck those between the liner and helmet. Finally, I wear a hat or my head warmer to get the final fit. It has worked really well, as I can attest from some of the rocks that I have smacked into. Probably more work that you want, but it does lay out one option.
BTW, I am also looking for an extremely small helmet with better coverage than my grateful heads helmet. If anyone knows of anything, let me know too! I have tried most helmets out there, but can't find anything that works. Because I have such a small head, I want to try it on first. This does limit my options to places that have a return policy. Thanks. (And yes, I am very small so I do not look like a peanut.)