From all three as is without context, A is the more interesting one since it picks curiosity and investigating the unknown while the others are more about hanging out.
I think they are overall good composition roughs but each has a very different purpose or target so in my opinion it would highly depend on the story.
•A feels more like its about a close encounter mystery/intrigue to the unknown.
•B feels more like slow paced friendly story for younger kids (far from exciting). It could work better if both kids had a sort of perspective that matches the alien creature and a more secure pose, maybe one of them even pointing forward or something. Or if you turn the kids to face the creature in horror/fear could also work as first encounter mystery but it depends on the story.
•C feels more like a wild outgoing exploring adventure (it gives out the good nature of the creature though... what I mean is if the story is mostly about the kids avoiding/fearing the creature for a while and then revealing it as being good then you wouldn't want to reveal that in the cover, something like the orange "eye" in cover A, there seems like it's going to be conflict around it but you can't tell if the entity is good or bad).