I am looking for a children's novel published anytime from late 1950's through early 1980's. The story is about some British(?) schoolchildren (assuming ages 9 through 13 here) that find a strange animal and believe it is an alien.
The animal is small and round and fuzzy; about the size and shape of a bright yellow furry grapefruit. (Kind of reminds me of tribbles from Star Trek.) It has no apparent limbs, orifices, or sensory organs. The children are unsure what it is at first, but decide it must be an alien.
When a local scientist/professor slices into it as an experiment, the animal grows to the size of a beach ball. The local scientist/professor gets it from the children and tries to bring it to a university for further study.
That's about half way through the plot. I don't know how it ends, and don't have further knowledge of it.