Childhood book, cannot find

Hi everyone. I'm looking for a book that I remember part of the plot, but not completely (obviously) nor the title or author. I believe it's a young adult book, possibly earlier than that, but the plot basically is there are two places, earth and somewhere else. When the full moons meet up on a certain river, it creates a portal. I remember there being moths, and I remember there being a leader of some sort ready to take over with robots. I hope you can help, I'd like to read it again, and I think my son would enjoy it.
M