Let's just make this clear to all parents. This type of shenanigan is not cute. No kid wants to be reminded of the truth about their conception. We all know the truth, but we don't need to stuff our faces with it, OK? This is just a bad way to spend your birthday. "What's this cake mean, mommy?" / "We're going to have a little chat, honey. I'll tell you all about it."
This is the type of cake you'd buy at a sexy cake shop. Hot tip for you: don't buy your kid's cake at a sexy bake shop. That could get you in trouble with a lot of the other parents if you end up having a birthday party for your kid. "What? Are you mad because your kid didn't get the balls? Sorry, but the birthday boy always gets the balls.