Honestly, since we have toddlers, I find absolutely no purpose to expand upon babies. I don't even keep them for longer than a couple days. Once my Sims' new baby moodlets are gone, I grow them up immediately. Then again, I never really cared for the baby stage anyway... not even in Sims 2 or Sims 3... and don't even get me started on Sims 1 babies (ugh)... lol
I'd much rather see teenagers be fleshed out. We definitely need a pack just for them (kinda like how Sims 2 had the teen style pack). Even if it's just a stuff pack, I'll be happy. I mean, we got 'Kids Room', so why not something for teens as well?