Oh man, that's not ruining them, that's just a classic combo. I mean, people put hot sauce or salsa on eggs all the time. Ketchup is just a sweet tomato condiment. It's the same idea. Your kid might come around to it later. Tastes change.
Yeah, "tastes change" is a big one... my buddy used to hate ketchup on everything as a kid. Refused to touch the stuff. Then he went to a cookout in high school, and someone put it on a burger right in front of him. He tried it, and now he puts ketchup on pretty much everything except ice cream. It's funny how something you think is gross just clicks one day. Eggs and ketchup are mild compared to some combos I've seen.