Filling in for a ten year old girl is kind of a weird concept. The fact that God is saying it, it might seem like T-Rex's consciousness is going to be in the body of a ten year old girl. My feeling though is that God just wanted T-Rex to be some kind of substitute, and go to the girl's classes at school, hang out with her friends. He'd be the dinosaur wearing a nametag that says "Natalie" or "Mackenzie" or whatever it is that ten year old girl's are named these days. If you ask me, that is funnier than if he had to fill in for a ten year old squirrel.
As the title text of the comic tells us (there is a Greasmonkey script you can get to read these in-line with the comic), Utahraptor is hoping T-Rex is going to give one of the old standbys of what God is telling him. As we have learned by now, though, T-Rex never has conversations like that with God. It's a bit of a running gag at this point.
It could certainly be worse. If God called on T-Rex to go out and murder some dudes, then he would (probably) be schizophrenic, and that shit is bad news.
And if God called on T-Rex to be a prophet, and T-Rex refused, it could ruin his life. I'm not just talking about the way Jonah was called to prophecy and got swallowed by a giant fish. Many people who feel called to prophecy may spend years of their life doing something else until they realize they have to start serving God, that it is the only way to feel satisfied with themselves. (I've talked about something like this in a previous post. I don't know which one.)
T-Rex is similarly lucky that when the devil talks to him, he only wants to talk about video games.