I remember a TON of details, but I can't for the life of me find the title of this book.
The story takes place during Earth's occupation by a race of red-skinned, black-eyed, three-fingered aliens, and involves an attack on the planet by other aliens that resemble Swiss army knives. That race's name starts with an H.
The protagonists are a young boy whose mother works for the chief of the occupying race and the chief's daughter.
It was a hardcover book. The cover featured both protagonists. The boy was in a blue shirt with a V neck, very 60s era Trek looking. The female protagonist was described as wearing a cloak and was possibly depicted that way on the cover. Her three-fingered hand was DEFINITELY depicted on the cover. And yes, the invading aliens were described as resembling army knives or multitools, but the exact term swiss army knife might not have been used. All of the occupying alien structures were described as open domes. The female protagonist mentioned that her ancient people lived on open plains ...while ancient humans lived in caves, and that's why modern humans lived in houses and this alien race preferred domes. This is a paraphrase of an exact exchange between the two protagonists (taken from comments. Ooops!)
I'm located in the US. I read this book in junior high between fall of 1996 to spring of 1997, but it was not a new book. If I had to guess, based on 20-year-old memories, it felt like, or I remember it looking like, it was published in the 70s. But I can't be certain.
The cover, on top of featuring both protagonists, may have also featured geometric shapes: cones, spheres, cubes. But I watched a LOT of UPN in those days.
Has anybody ever heard of this book?