Height sliders, but I have a very specific request for them. I don't want just completely analog height sliders. Just have, say six set heights: Infant, Toddler, Very Short, Short, Average, and Tall.
Infants can be infant height. There's no real need to have super-tall babies. Same for toddlers; just toddler-sized.
Children can be Very Short or Short, with the default being very short.
Teens default to Short, but can also be Very Short, Average, or Tall.
Young Adults/Adults/Elders can be any size from Very Short to Tall.
At child, teen, and young adult age-ups, you can choose whether to make your sim taller (but not shorter without cheats), or the default will be Infant, Toddler, Child (Very Short), Teen (Short), Young Adult (Average), and then remaining average height.
Why do I want this, which seems way more complex than just a height slider? For the animations. If you have a very short sim, regardless of age, they'll do a task the same way (if it's appropriate for their age). If you have two sims of different heights interacting, the game checks the height difference and uses the appropriate animation for the height difference. A tall sim shaking hands with an average sim could use the same animation as a short sim shaking hands with a very short sim. It means you need multiple animations any time a sim that could be multiple heights interacts with another sim or item, but I think it's the best way to do different heights and still be able to line up animations properly.
What made me think of this was installing a mod that let children do adult exercise activities. Occasionally they'd stop and stretch and because there wasn't an animation for child-height sims stretching, they'd suddenly turn into Slenderman, and then back to child-sized.
And if that causes a need for too many animations, you could reduce it. Maybe teens through elders only range from short to tall. It reduces the option to make sims who are little people, and I'd prefer to have that option, but there are probably programming and other considerations.