Frostlings, Emberlings and Sparklings do not take damage in combat.
I'm sure that the three units I mentioned above do not take damage in combat. I'm new to the game, but I took special pains to make sure I wasn't mistaken in this observation. So far as I know tonight was the first time I noticed the bug. It also was the first time I had some really long and big battles in My Pet Dungeon, Frostfossil.