Hot water in the floor would be my choice . . .
The storage deal, well, if a little one is sleeping in that room (or anyone for that matter) you will want even heat - all the time.
Now, here is what our electric co-op suggests/offers for floor heat. A guy down by the lake put the low voltage grid in a guest cabin he built. It does work pretty good . . .  http://www.electricunderfloorheatingsystems.com/