story identification - What diary-style book has a family flee destruction of earth by exploding sun and try to survive on a new planet?

I’m looking for a book I remember. A girl her father and brother (?) leave earth on an ark ship headed to another planet. After several years (?) they arrive on the new planet, but it is much harder to survive than anyone thought.

I specifically remember crops not growing the way everyone thought they would and their animals dying. Wheat is crystalline and practically inedible, etc.

There were also some kind of moth people.

The kids end up helping everybody because they are more adventurous than the timid adults and try new foods etc.

They get through the first year and realize they should be recording their history in some way.

They find out the girl had been keeping a diary and begin reading it aloud. The first sentences are the first ones of the book.

Answer

This is probably "The Green Book", a 1986 novel by Jill Paton Walsh as per "Story Identification: colonists on a planet where the wheat is crystalline"

Pattie and her family are among the last refuees to flee a dying Earth in an old spaceship. And when the group finally lands on the distant planet which is to be their new home, it seems that the four-year journey has been success. But as they begin to settle this shining world, they discover that the colony is in serious jeopardy. With supplies dwindling, Pattie and her sister decide to take the one chance that might make life possible on Shine.

Indeed, the wheat comes out crystalline and is poisonous to the humans and livestock. The settlers survive on rations, but are close to running out. The moth people hatch from what was originally thought to be a hill of boulders and they eventually return to those round objects.

The children realize that the wheat can be eaten if it's ground up first if I recall correctly. I don't remember how they figured that out.

And yes, the girl has been writing up the history of the story. Due to storage limits, everyone was allowed one book (later in the book, it's established that there are a lot of duplicates, and those who brought a unique book charge high in barter for the privilege of borrowing it). Her siblings teased her unmercifully when they first set off because she brought a blank book, but of course, it turns out she's been writing the first new book of the planet.