I would leave this to kids who can read without zhuyin already and knows at least 1500 characters or more. There aren’t any hard idioms in here but there are references to classics like Charlie and the Chocolate Factory, Tarzan, etc. so it’s for the older child for at least 3rd grade or a very mature 2nd grader or even a very very very mature first grader.
It’s a very humorous set translated from the original Spanish works and talks about a little boy with bad habits like most children (waking up late, forgetting things, getting into accidents) and he meets these little monsters and the only way to keep them alive is to read to them. In that journey, he reads these classics and the classics are given a summary at the end of a book.
I like this set due to all the pictures it has! Look at it! It makes reading Chinese a breeze. I personally love illustrations so I buy books with as many illustrations as I can find.