Well to tell you guys one thing about the swear words on the childrens shirts, in Japan, they don't have swearing. The meanest thing you can say to a person is "go die, idiot" or something similar. There are no insults "exept for pussie, which is 'manko'" that you arn't allowed to say infront of your parents like fuck, shit, etc.
So the t-shit makers probably thought, "hey, lets right idiot on the shirt", and it came out as FUCK YOU! or something. They don't know what inappropriate words are.