Could this be the Matthew and Maria Looney series by Jerome Beatty Jr....? There are a total of seven books in the series, published from 1961-1978.
The Matthew Looney books chronicle the adventures of a brother and sister, Matthew and Maria Looney, who live in the town of Crater Plato, on the Moon. In Beatty's stories, the inhabitants of the Moon are a fully developed non-human civilization. Beatty's fictional Moon inhabitants are an indigenous species, living on the Moon without the assistance of spacesuits, "breathing" vacuum instead of air. A recurring theme in the books is Matthew's desire to know more about outer space, especially the Earth. At the beginning of the series, he looks up in the sky at the Earth and wonders if anyone is living on it.
Similar to most child characters in children's literature, the Looney kids must deal with parents, friends, and rivals, along with the normal array of school-age joys and concerns. Their father works in the "powder factory," but Matthew is captivated by the notion of an exciting career in the military like his uncle, Rear Admiral Looney. Maria, his younger sister, prefers playing sports after school and getting in trouble with her best friend and rival, Hester.
During their adventures, the Looney kids face space pirates, a war between the Moon and Mars, the "discovery" of life on Earth, and the invasion of an Earth circus.
This article offers a decent overview of the series.
Jerome Beatty Jr.’s Matthew Looney’s Voyage to the Earth (1961). With endearing illustrations by Gahan Wilson, this now-forgotten classic of YA sci-fi tells the story of a boy from the Moon who wonders if (scientific consensus to the contrary) there is intelligent life on Earth — and who, despite bullying, political intrigue, and danger, stubbornly sets out to find out the truth. The Moon’s civilization, it seems, is almost exactly like our own, except for that fact that they eat scrambled arks (and burgles with canal juice), measure distance in lunacules and time in moonits, and must be very careful not to jump too high… and go floating off into space. Matthew’s adventure is set against an all-too-Earthlike backdrop of space-race political maneuvering: While scientists like Professor Ploozer stress peaceful and scientific uses for their rockets and missions and probes, the Moonster military and politicians are more interested in developing and testing weapons — which they’d like to use to colonize or destroy the Earth! Fun fact: Subsequent Matthew Looney books include Matthew Looney’s Invasion of the Earth (1965), Matthew Looney in the Outback (1969), and Matthew Looney and the Space Pirates (1974). Matthew’s sister, Maria, is the protagonist of Maria Looney on the Red Planet (1977), Maria Looney and the Cosmic Circus (1978), and Maria Looney and the Remarkable Robot (1978).
The sixth book in the series is titled Maria Looney and the Cosmic Circus (1978). Here's the synopsis.
The arrival of Bill Bailey's circus on the moon means trouble for Maria Looney, especially when it becomes apparent that someone is trying to steal the top-secret designs of the Moon's L-Bomb.