OOC: A Guide To Your Batman (Ages 9+)
First up: What Canon Are You Even Firing??
Bruce is taken from just before the end of the Justice League Unlimited episode "Kid's Stuff" -- you can all kill me later.
Second Point: Excuse Me, But Your Plotholes Have Collapsed The Roadway
Bruce is an adult who was supposed to be temporarily transformed into a child in order to fight a child who had rid the world of all adults(Yeah, I know right?). As they all retain their adult memories, and abilities you'd assume they're basically adults in child-form. You'd be wrong, though!
If you watch the episode you'll notice Batman doing things he would never, ever do' whine "It's not a raaaaace!" or use "whatever" as his totally awesome comeback of choice. So. This leaves me with the problem of 'is he and adult or is he a child?' to deal with.
The answer to this is, very simply (but not at all simply), 'both'.
My Bitty Bats is actually much more mentally developed then his canon would dictate that he be. This is because I cannot, ever, imagine someone who was (and still retains the knowledge of) an extremely intelligent detective who fights crime for a living to suddenly loss all of that and just be a nine-year-old who happens to remember being Batman. Because that would be stupid (Uh. Ignore the fact that I'm playing a canon nine year old Batman).
However, I also can't imagine someone who is a nine-year-old (even with adult memories) who can maintain the same sort of focus and sheer dark anger as Batman does. Because childrens' bodies don't allow that. They have much less control over their actions and this isn't just because they haven't learned how yet it's because their body chemicals are very, very different.
The longer Bruce stays the way he is, the more he will give in to his own body's demands of him. The more stress he's under, the more child-like he'll act. And most certainly, the more people treat him as a child and he sees himself (in mirrors or just looking at his hands for instance) the more he will relax into being a child. This isn't to say he won't fight his own regression tooth and nail! It just means he can and will temporarily slide into a mind-set more indicative of someone not over fifty.
He'll never fully be a normal kid (his parents are dead for one and he's lived an adult life for another) but he will have very child-like reactions to many things.
Third Point: Your Batman's Lips Curled And Not Downwards Either
That is because Batman is human. And it is my firm belief that, though they sometimes might not want to, all humans who are not brain-dead or in a coma smile.
Besides animated comic canon once had him making pancakes for his faaaamily~ in a kiss the cook apron and serving lemonade so I have some sort of (cobbled together) leg to stand on.