This is the least kid-friendly of the Frog and Toad books. Lobel was clearly undergoing life changes by this point in his career, and things don't always come out as innocently as they do in earlier installations in the series.
For example, in the kite story, some decidedly mean-spirited things are said—things that are difficult to explain to a toddler who has never experienced teasing, insulting words, and the like. While the other books in the series are rated "G," this title is closer to a "PG" rating—it will require some amount of work on the part of parents to explain it to kids that remain reasonably innocent.
At the same time, it's still Frog and Toad—the characters haven't changed, and neither, ultimately, has the wisdom involved in these stories—it's just that in this volume, there is a greater mismatch between the reading level and the kinds of wisdom being presented.
It's not an egregious difference—and we still read these stories to our toddler, just as we have with the other titles—but the difference is there, nonetheless.