I recently discovered that The Last Mimzy is not, in fact, the adaptation of a similarly titled book that I vaguely remember from childhood. So what was that book?
I think it was titled either "The Last [blank]" or possibly "The Great [blank]", where [blank] was a made-up word reminiscent of Mimsy.
I read it in 1993 or '94 (third grade). It was published before then (i.e. it wasn't a new release that year). It was written for a slightly older audience, but not much older. I used to think it was written by Roald Dahl, but it isn't anything listed as his. I remember thinking that it was similar to E. Nesbit's books, but it isn't hers either, and I don't remember her books well enough anymore to be confident in that.
The plot, as near as I can remember, was that two (or more?) children were taught by an older woman (a tutor or governess, maybe) about this [blank], who was some sort of sentient creature that was the last of his kind. Then they learned some sort of skill (I think some unspecified advanced mathematics, but it might also have been dream-based?) which allowed them to teleport to the see the [blank]. I recall the [blank] was small, and I think it was some sort of leader in whatever region it lived in. It taught them more of the same skill, and the youngest child advanced much faster than the POV character. I think the POV character (she?) eventually caught up, much to the younger brother(?)'s delight. I think they eventually decided they preferred to go home, and had to escape the dreamland/alternate reality/whatever.
I think this book may have used the word "tesseract," but I also read the Time Quartet the same year, so I may have blurred that together.
The book was a full novel. I remember it being larger than a trade paperback and something like 3/4 inch thick (no idea on page length or word count).