So while kicking the noun to defecate out of somebody's noun donkey doesn't sound nearly as bad and doesn't have the same ring to it, it will confuse most seven year olds, and that confusion might be worth it if you don't want to have to cover those younger ears when ever you want to talk about You-Know-Who. But before you go out and start talking about defecate, donkeys, and female dogs maybe you should just go wash your mouth out with 72 bars of soap.
Speaking of things that young kids do with a dictionary my favorite prank was to tell people that they needed to go look up "handsome", "polite", or "clean" in the dictionary because they would find their picture next to the listing. To properly prepare for this gag you do need to put your best seven year old art skills to the test to create a proper likeness of your prank target that would look like the popular presidential portraits and remember to label if your artistry isn't up to par. I found that post-it notes work well for this prank because you can attach them to the page and the adhesive won't tear the page or leave smudges when they need to be removed. Granted, I didn't put the post-its next to those specific words and nobody actually would look up those words I did tell them to look up, but it is the though that counts.