I'm talking about at least two juvenile sci-fi novels, read between second and fourth public grades (U.S.A.) which for me places them around 1956-9.
The protagonists are a brother and sister, presumably my (then) age. They are friends with a single man, a scientist who has built a rocket ship. The scientist is little fey, physically; a (relatively) large, round head, balding, short in stature and striking long, thin hands and fingers.
The rocket ship's final preparation is to slop a viscous, clear gloop over EVERYTHING, inside and out from a pail with a paintbrush. In flight, there is an encounter with the moon, and a very small planet with (non-threatening) inhabitants not physically unlike the fey rocket scientist. The planet orbits either the moon or the earth, not the sun, and strangely, some kind of mushroom(!) theme runs through the books. Really.
That's it. That's all I got (I was nine at the most, fer chrissake). My recollection tells me there was concealment from parents, a particular name for the planet and people and flight through a spatial fog. I mention these things separately because I am less sure of them than the previous two paragraphs. The author was blissfully unrepentant of the laws of physics: Earth gravity on a micro-planet, fog in space and super-gloop to ready the ricketiest rocket. Worked for me.
I'm guessing even Richard, my previous mentor/savior might not be familiar with these. Anybody? Author, titles? Or do you just recognize one of these?