Protagonists were Sally and Jim, ages 11 and 12 or thereabouts ("Jim, six and a half, and Sally, six" in Martian years) middle-class white suburban kids whose family moves to a Martian dome-city.
Previous colonists believe the highest Martian life-form is a three-eyed tortoise (possibly with some discussion of how Earthly animals' pituitary glands evolved from third eyes), but Jim and Sally discover a cute Martian cat and humanoid Old Martians. Illustrations include said animals, the dome city, caterpillar-tread domed rovers.
Probably from the 50s or 60s, early 70s at the latest? Paperback cover possibly yellow.