As long as I've known Truman to talk, he has called Harrison by the name of "Ya-ya." That's cool. Tru isn't the leader in the toddler-speech field and actually hasn't even mastered the shortened (let alone full-length) version of his own name.
So, I'm used to the Ya-ya thing. Not exactly a fan, but what IS one to do? And truth be told, I've caught myself even using the misnomer every now and then. The other night though, Truman really took it to the next level as he asked for help getting the last firefly out of his jar. I was just close enough to catch: "You do it, Ya-ya James, you do it."