I used this book for the first time with a kinder group awhile ago and it’s wonderful! Not surprising, because Jayneen Sanders wrote it and her books are great.
I used it as a model for how talking about (or drawing about) our worries can make them smaller. It felt spot on developmentally for the kinders, though I’d use preK and 1st also. After my group read it, we drew pictures of different things that make us worry (specifically, “The worry monster visits me when…” because we used the Monsters Feelings group).