What would happen if I aged up toddlers b4 maxing their skills?

It won’t harm them at all. They will be able to walk and talk. I think toddlers with maxed skills might start grade school at a slightly higher grade level than ones that don’t. But that’s super easy to overcome once they start school.

If you have Parenthood installed, that’s the pack that will really impact your kids later because based on how they’re raised it will affect their interactions with other sims once they’re grown as well as job and uni performance. Relax and have fun experimenting with both packs. Just like real life, maybe not all of your sims turn out to be perfect people.

I haven't noticed any issues on the few saves I have where I've just aged past the toddler stage. there was one (and only one) who I did play through toddler and didn't get his potty training finished. As a kid, he seemed to wait a little longer before autonomously going to the bathroom, but he never wet himself. All other traits were fine. The worst that happens is that you actually have to start down at 1 for the child skills (Mental, Motor, Creative, Social) rather than just shy of 3.

You can max parenting starting at kids, but it's easiest if you start at baby and work your way up. Easiest, but not required. Especially if you have the parents more hands on than I tend to do. I've yet to have a parent max parenting if they start with teens, though, and I play on long life.

The primary thing playing through kids gives you is a head start on some adult skills once you've maxed the child skill, and a head start on Character Values.

You don't need to start as a baby to succeed at University. I ran a testing sim that I'd started as a young adult through there (that, admittedly, I'd played for about 2-3 sim weeks before I got Uni) and she came through with straight A's. (Granted, I focused on getting through her classes, so she didn't have the full Uni experience.)

Check out this link for some good toddler info! https://www.carls-sims-4-guide.com/parenting/toddlers.php

By maxing out skills, you can gain a headstart in child skill building and earn some extra traits that speed up future skill and career gains. You don’t acquire any negatives if you don’t max out skills - the Sim kid can still talk and walk. There’s really no dramatic downside to not maxing out skills, you just won’t earn the extra bonuses.

This might also be of help. It talks about some of the character values you can earn while growing up: https://www.carls-sims-4-guide.com/gamepacks/parenthood/values.php