I'm sick of having good, athletic babies, guys. It annoys me sooo much that born-in-game sims are doomed to such a lack of trait diversity, especially if I'm not playing them. Ideally I'd have every trait available to me upon birth and onward but the only mods I can seem to find only unlock a few, and those mods had broken links anyway so I couldn't download them even if I wanted them. If anyone knows where to get my hands on this it'd be much appreciated.

Over at the Asylum is this https://simsasylum.com/tfm/index.ph...ebruary-2-2014/ . You do have to register there but there are alot of great tuning mods there and they are all up to date.

I use Nraas Retuner settings to set traits at 15, I then use Master Controller advanced settings to change traits on Sim.
I mean, that's a little more work and having to read a bit on how to understand and use these mods than simply sticking a package in your files, but it does give you a lot more choice to set stuff and control things yourself.

I have mine set to up the number in retuner to 10 also but I think what OP is asking for is more available for the age stages to choose from not just be abke to add more. Some traits are locked to certain age stages like you can't select any of these for babies:
Adventurous
Bookworm
Can't Stand Art
Computer Whiz
Coward
Daredevil
Family Oriented
Good Sense Of Humor
Hopeless Romantic
Hot Headed
Lucky
Mean Spirited
Natural Cook
Neat
No Sense Of Humor
Perfectionist
Photographer's Eye
Unlucky
Charismatic
Shy
Diva
Natural Born Performer
Night Owl
Loves To Swim
Not sure if retuner can unlock that option although I would think it should. It can be an intimidating mod for people though. Hmmm I may have to poke them over there and see if anyone has tuned that through retuner, I want a Diva toddler now...that should be fun, or a Mean Spirited toddler.

Could be mistaken about this, but even if those traits were unlocked I don't think they are going to have any real impact on sims too young to have the trait-defined effects imposed upon them. How exactly is a Hopeless Romantic baby or a Computer Whiz toddler supposed to behave? I understand the desire though, since of course these traits will play more of a role in defining the sim once they are older an without the player having to step in with MC as often and rearrange things.

I don't really mind that the traits won't impact gameplay until they're older. Until then I can always direct the child to act in certain ways that would allude to their traits myself. It just really bugs me that there are certain trait combinations you cannot get if you have a child in-game. Like I'm working on a legacy challenge right now and picked out some traits for the generation themes, come to find when my heir is born that none of those traits I picked are available to babies. The Midlife Chris reward isn't available until young adulthood either so I'm just stuck with a couple of traits I don't want and have to spend a chunk of lifetime reward points later to fix it. It doesn't help that a few of those traits I don't really use, like perceptive, insane, disciplined. The mod Mandelyn posted helps a lot though.

If it's your active sim, just enable testing cheats and then shift and mouse click on your sims for the option to Modify Traits. The trait windown then pops up for you to select/remove.