Does anyone know how to change the name of a child sim? I wasn't really paying attention when he was born and I gave him a girl's name by mistake. lol.

In the cheats window:
testingcheatsenabled true

shift + click on the sim (if he's still a burrito you can't do this, only if he's a toddler or older)

"Edit in CAS"

And then you can change his name, gender, whatever. Just don't mess around with his age or species (from what I hear that can get weird.)

at city hall

You can also do it with MasterController.

Just a note, the parent needs to take the child there between 8AM and 6PM if you go that route.

That's what I was doing wrong! thanks!

It does get weird if you mess with the age; you end up with a corrupt, stretched hybrid between the two ages, at least if you try to turn a child into a toddler. However, I aged the toddler back up into a child through CAS and she's worked like a charm so far. I've never tried messing with species. And yes, you can change the name in CAS but it's much quicker to do with MasterController, trust me. (I think MasterController can also optionally update the whole household's last names and the household name if you edit a sim's last name.)

I know about MC, but I figured I'd explain the base-game way first since MasterController is kind of big and confusing to navigate at first, and if someone doesn't already have it they probably won't want to install it just for one thing that can be done without any mods at all.

I just wanted to add: Unless you have MasterController. I do, and I edit my town's babies in CAS all the time, usually to change their names.

Basic... Rename is much quicker than Edit in CAS. FYI.

That could be one of the ways, but if I were you, I'd avoid using edit in CAS cheat as much as possible.