Home is where the heart is...
It can feel super hard to get your pre-teen to show emotions OR EVEN BREAK A SMILE without thinking it's gross and not cool. I get it, I DO remember what it was like being a teenager and I also remember how much one might get teased for having emotions (god forbid you do *rolls eyes now at my teenage years*).
And you know what the best piece of advice I could give you for your session is, if you have kids (or even...as adults)? Let them be themselves. Give them the permission and freedom to just be whatever they need to be in that moment. I know, I know. Getting that smile and having them "look" normal might be what you want...but it's not what they need. Let them get their energy out, their sillies. Let them own their personalities and show it how they want because that's what I want to see (and you too. if you're honest with yourselves). And well, you know they're going to do it their way anyway, so let them! Bonus in that it also helps them loosen up while I'm there and it gets them comfortable with me while they get ! Let them show me their personalities!*
* I totally also get that there are also kids out there who are more stoic in general or not normally smiley and that's okay too! It's probably something you're used to already (and might've already warned me ahead of time LOL) but I am always there to gently coax everyone in front of my camera to have fun and/or smile in the most natural ways ever! Typically involves me being myself (aka cool but silly)...or so I believe I am lol