it seems silly that something a child would come up with on his own can be considered hugely offensive.
Once as a four-year-old I was attempting to cling to my mother's leg in a pool and I stumbled upon the nonsense word 'nggr!'
But you didn't attach it to a black person, did you? We have Brit and Scot, so Jap seems like a logical term for a person from Japan. Kind of like Paki for Pakistani, when we call people from Afghanistan Afghanis instead of Afghanistanis.
Didn't you get banned over this?

Playing with fire if Kara stumbles in here.
See, I agree, they are reasonable contractions...until at some point they're weaponized and turned against the group in question. That's happened to 'Jap'. It was used in propaganda posters, broadcasts, military slang and common parlance to designate a slant-eyed, rice-eating hive of interchangeable and fanatical yellow-skinned supermen who wanted to conquer the world and sexually violate American women. That's what the term means,
especially for people who lived through that era.
Whether you are a Japanese national today, or - worse - an American citizen who was treated as a traitor and a 'Jap' just because of your ancestry, you don't want to be that any more. You want to be a human being, not a monster.
Jap isn't just a contraction any longer. It was repurposed. You can't pretend that it's short for Japanese any more than you can claim that 'negro' or 'colored' are acceptable terms no matter how factually accurate they might be.
(And I'm gonna guess the same goes for Paki.)
The same thing could happen to Yank one day, and when that day comes it'll no longer be an acceptable term for Americans.