I need help finding a fictional book series i read as a kid. It was called Netherworld or something?

It was either netherworld, or neverworld, or something i cant remember. Ive searched for it but i cant seem to find them. It was something world, anyway each book cover had the same gold symbol around the eyes of watever was on the front of the book. It was about the kids who go to this different world where theres gods, and aztecs, and knights and stuff. Then whenever they fall asleep their back in the real world. Anyone have a clue as to wat im talking about?