Although some boys switch it up a bit, Timmy Turner sticks to his briefs through and through. Not sure which episode this is, so I have to rely on my fantastic audience once more.

Update: The episode is from the special "Wishology: The Exciting Middle Part"

Timmy Turner gets quite a few wedgies from his horrible babysitter Vicky. Going by his expression in this scene, he's just used to them now. I found this screenshot online, so I don't know which episode it is.

Update: The episode is called "Vicky Loses Her Icky"

Here's another show that just loves embarrassing it's main character in his briefs. All of the screenshots I've found for this show have been created by other people and it's been a long time since I've watched this show; so I'm afraid I don't know the episode for this and other Fairly Oddparents screencaps. If you never watched the show however, pick a random episode and chances are high you'll see Timmy Turner in his skivvies.  

Update: The episode is called "Chin Up"