I'm just...
You get three days (on normal lifespan) to play babies, so any goals or objectives for gameplay would have to be something possible within that timeframe. And I personally would prefer it to be something I didn't have to work on to the exclusion of any goals or skill learning I might have in mind for the rest of the family.
I mean, It's not a bad idea, but I can see it being really frustrating if you basically have to have an older sim tending to the baby for the bulk of the 72 sim hours that they're in the baby stage in order to achieve those milestones. Possibly to the point that only the family/baby players actually pursue it more than once (unless the mechanical benefits are clear and significant - like the toddler skill starting at 2 or something.)
I play on long life, so the milestones would probably be achievable for me - but I'd have to see how they were implemented to see if such a thing was actually *fun* for me. (I recently had a household with 6 toddlers/children at the same time. I learned then that I am never, ever, going to try the 100 baby challenge because I hated it. Not my definition of "fun" at all.) I also get really tired of the baby crying track and tend to age up babies quickly because of it.