I haven't read all the posts in this thread, so sorry if this has already been mentioned a thousand times
I usually like playing with families, but the amount of toddler interactions is disappointing. I'd love to have more things for them to do. I never played The Sims 2, but I've heard quite a lot about toddlers being able to ask adults for attention, for food, etc. I'd like it if Sims 3 toddlers could do that - if they've been taught to talk. I'd also love to see toddlers who can walk escaping from the crib.
More interactions between toddlers and between toddlers and their child-aged siblings would be nice, too. I've got the toddler interactions mod, but even with that, there aren't really many ways for toddlers to socialize.
With regard to babies, I'd also like for them to be able to actually
do things. Be bathed, wear clothes, or at least make it possible to change the colour of their blanket to customize them a bit, maybe have a parent sing to or rock them.