A Highlights rebus never has more than 17 lines of text. Watch out for too much dialogue in your story because it can eat up precious lines. Sometimes a retold folk tale works well in this format. It was the first day of school. Small wins over big.
Kids figure things out on their own.
Stimulate your readers' imagination. Avoid negative words. You can even teach a lesson through a rebus. Teach a mini science lesson, how to count money, how plants grow, changes in the weather, a craft activity, or something about another culture. Don't use puns.