Toddlers in general. I'm not into playing stories that revolve around babies or toddlers. So for me, their entire existence involves random needs fulfillment and if I feel like it, skilling to get some bonuses. I think they're a necessary part of the game, but I can't personally play them, so I either suffer through it or spend a lot of time taking the parent(s) out of the house on excursions. I like legacies and seeing a family evolve, though, so I need to deal. I could age them up early, but I like each lifestage to be as long as it's designed to be, I don't want to shorten anything or throw off a child's age in comparison to the age of the parent.
So I suffer, or the parents take a long vacation.