Toddler and infant ideas
Here I wanted to give ideas that wouldn't be hard to to do plus it might spice up these life stages a bit.
Toddlers
When Toddlers are potty level 3, sometimes they feel proud of their potty making them happy. So my thought is to remove the use diaper interaction at level 3 potty skill since they never like using it and instead count level 3 potty skill toddlers using diapers as a bladder fail with no change but it will make them feel sad as in they missed the potty. Gives them more life as in they love the potty so much and hate diapers with their skills.
Next I wanted to give more auto actions with toddlers interactions to others. They never do the talk about interactions on their own instead they mainly do funny actions like joking or singing. So them being able to just talk about stuff on their own could give more fun life to toddlers when talking.
Lastly I feel like Toddlers are a lacking playground items in sims 4 and with npc toddlers showing up at playgrounds without a slide. Since slides are considered base game for toddlers now. Maybe a base game slide would give something nice for toddlers on playground lots.
Infants
Infants can't do much in sims 4 so the first option is a base game changing table since they are considered base game.
Next up is a family in basegame with a infant that would be perfect to have a infant like the pancakes family with iggy the toddler went toddlers were added. Maybe the spencer-kim household with a new sibling for Olivia.
Lastly Infants could use some more clothing and hairs in the game. Mainly hairs but make some of the editions base game since it could really fill in choices for those little ones since they missed so many packs and even a infant related stuff pack would be nice too.
Hopefully you like these ideas.