You aren't the first one to do this. He just slept it off, and remember it's only a 4 hour dose, which really is over after about 3 1/2 hours. Pain medication never last as long as it says. (If you have ever had surgery and used paid meds, you know what I mean).
Long term, don't worry. If you did it over and over, yes, you'd create a problem for the little guy. Once is a mistake for the day, not forever. Don't beat yourself up. I'm a mom of three and have worked with children for years. And what I found to be true is that, you will make more mistakes. The key is to realize when you do and learn from them. Happy Holidays!