I'm trying to find a book I read as a child/young adult in the mid-70's. The book involved a father (or grandfather?) building a spaceship behind his house. The father and two children (boy and girl) took a trip to Mars where they encountered two alien races at war with each other. The alien races were vegetable-like; one (good?) was short and squat , and the other (bad?) was tall and thin, cucumber-like with tentacles. The aliens communicated with the humans and with each by telepathy. I borrowed the book from the public library in our small town in upstate NY. The book had illustrations; I can still picture the spaceship and the aliens. I would guess it was written in the late 50's or 60's, probably before manned missions to the moon. I know this probably sounds like dozens of other books, but maybe someone else out there read it and remembers... Any help will be appreciated.