I think of toddlers as being about 2-5 years old. But that's well into the stage where they walk (my sister was RUNNING by the time she was one) and talk and have conversations. I was already a bookworm by 5 and it's bothersome that a sim toddler can't have that trait.

Which is funny because I can make them evil.

And I think the game tries to standardize child development too much. If parents and other adults around couldn't be bothered teaching their toddlers to walk, talk, and go to the potty, then the children, teens, and YAs they grew into shouldn't automagically know how to do those things either.

At least, that's how it works in my family.

I'd call a 4-5-year-old a small child rather than a toddler, really. As far as I'm concerned, "toddler" refers to the age group between 1 and 3 where humans are typically still in the process of getting used to speech, upright movement and not soiling themselves, not pre-compulsary-school-age people that were born more than twelve months ago in general.

That being said, you're right about TS3 toddlers being far more limited than they logically should be. Even after being taught all three toddler skills, they never seem to advance that far beyond the "crawling infant" developmental level they start out at- The Sims Freeplay handled this much more effectively, given that toddlers in that game could interact with far more household objects and seemed to remain upright unless left idle for a while. I could go on at length about how to design a better toddler stage- I've occasionally thought about a fairly detailed outline for one while bored- but I doubt anyone in a position to do anything about it would listen to me even in the unlikely event that any of them lurk here.

Why do plant sim toddlers need to learn to go potty or they get a random trait when they age up to child?

Plant sims don't go potty.

They need to understand how the skill works in case they ever adopt non-plant sim kids and have to teach them?

I've considered toddler to be one or two year olds and they age up into a child on their third birthday.That means it takes a baby 180 days to get past the baby and toddler stages before they become a child.

Sims 3 toddlers are almost the perfect kind of toddler.
They don't scream and cry because you won't let them stick their finger into an electrical outlet.
They don't slap, hit, bite or claw.
They don't draw all over your car, floor, door, washing machine, fridge, and their own face with a permanent marker that grandma gave them by accident.
But they also won't love on you after throwing a 20 minute tantrum because you tried to wipe said marker off of their face.
... Yes I speak from experience.

I think what EA describes as a toddler is a weird mixture of real life infant and toddler. You don't really have to DO anything with them, other than feed and change them and occasionally interact with them, much like an infant. But you have to potty train them, and teach them to walk and talk just like a toddler.
I don't think anyone at EA has ever had one before.

I cannot help but immediatelly I thought 'bout this as an actual set of modding ideas...