This type of planning made the execution of this project go much faster and only really observant kids (and adults for that matter) will ever notice that some of the elements repeat. Although this example is the most obvious repeat, it made total sense in the context of the book, so I was okay with it.
It's tempting to mine your digital archives for things that might speed up a project, but whenever I do reuse elements, I try to disguise them as much as possible. This includes, cropping, changing colors, flopping, rotating and more to make the old element new and to integrate it into the new piece. I only do this when I am either, short of time, short on budget, or both, but it's one more trick that can be helpful if not abused. Don't get sucked into the habit of constantly using shortcuts though, or your art will suffer. There's still no substitute for good drawing and original thinking.