Warm & Plush. It's the thickest 100% cotton batting I've found anywhere. I don't use wool for little ones because some have a sensitivity to wool. I don't use bamboo rayon because of all the chemicals in it. Poly could be fine, but I've personally found that there are many parents that are nervous about the fact that it melts when it catches fire (yes, that's unlikely to happen, but not having kids of my own, I try to honor parents' concerns). The other nice thing about Warm & Plush is that it's heavier than poly so it doesn't move around as much. My own nieces & nephews loved it when they were little.
If possible, I'd still recommend using the quilt on top of an area rug, rather than directly on the hardwood floor. The more padding the better.