DD is still happy with her Wilson Pro Stock. It's fits her round melon. My DS has a UA (All-Star made) helmet he's very happy with, but his head is longer and more lean in the middle. They're the same age, so it's always interesting to see how differently each one's gear fits the other(her shins and CP are longer than his as well. No sharing equipment at my house). I found a good deal on a used Large Wilson FX, but it's significantly larger than the small/medium size. I don't know that she'll ever grow into it, but I'll be using it to warm up pitchers. I'd definitely go scout some brands out before the inevitable happens.
*Edit* the Pro Stock that DD likes is not the C1K helmet pictured above in this thread. She tried a LS helmet that looked very similar to the C1K, and hated it.