I've got a bit of thing about toddlers being 'sad' and older Sims being 'tense' when 'scared' is the proper emotion. I agree, though, that it would be fairly difficult to implement - you would need to change the moodlets generated by dozens of interactions and circumstances, and then tune them appropriately.
We've seen that tuning can be a bit of a challenge with Sims laughing themselves to death over clean laundry or going into a two-day funk over the death of a relative they've never met. I don't mean that sarcastically - in some cases, other moodlets will mask the negative moodlet. I never had a laundry death, and the mourning moodlet seems to work well for the adult in my house - she is occasionally 'very sad', but perks up when she picks up some positive moodlets. That's fairly realistic, really. It doesn't work as well for kids and especially toddlers because they don't get some of the adult buffs, and go into a funk for hours.
You don't want to under-tune, either, because then the moodlet is overwhelmed. It wouldn't make sense for a Sim to go back and forth between being 'happy' and 'scared' - if they're scared, they should be scared until the scary thing goes away, or, in the case of kids and tots, a parent comforts them. At the same time, you don't want your adult Sims scared to death because the moodlet is too powerful.