I would be a little worried about them picking up the gravel curiosity wise and potentially getting it stuck. If you use something like pea gravel, I would put sand over it rather than just the gravel, but you still run the risk of them eating the sand maybe? I would just go with all dirt and...
I leave the kiddie pool in there throughout the winter since sometimes there are days in between freezing that there is a thaw. Sometime I might dump hot water in it to melt the water if it's going to be a little warmer to help melt. I have a couple of heated water bowls inside throughout...