story identification - Children's novel: three children (plus dog?) travel to the Moon in pretend spaceship turned real

I remember a children's novel that I read in the late 1970s / early 1980s about three children travelling to the Moon. There were some illustrations; I think the colour scheme of those was black, white, orange, and purple.

Three children (an older boy, an older girl, and a younger girl, probably siblings? plus a dog?) are playing let's pretend in their back yard. They build a spaceship from cardboard boxes and a bucket. They intend to name the spaceship "Moonbeam", but the younger girl misspells it as "Moonbeem" when she paints the name.

Overnight, a wizard who needs a real spaceship transforms the children's pretend spaceship into a real one, but forgets to change it back when he has finished with it.

The children discover the now real spaceship the next morning, and decide to travel to the moon. The older boy is piloting the spaceship, and at first, he has difficulty controlling it and flies under London's Tower Bridge.

After they land on the Moon, the younger girl has difficulty putting her spacesuit on and needs help. On the surface of the moon, the children rescue a Martian who has suffered a spacesuit puncture.

On returning to Earth, they use Tower Bridge as a landmark to find their house, and the older girl reminds the older boy not to fly under it this time.

Does anyone remember the title and author of this?