I don't see how this would feasibly work, nor do I see how this would be efficient.
There are two separate models for the babies and toddlers, so I don't even think they could possibly be merged. Like seriously, how would that work? In the beginning they have the baby model, but then as time goes by, they become the toddler, yet the entire stage is just called baby and there's no aging up involved? Sorry, I don't understand. It sounds very awkward and confusing. It would also be extremely weird if the 3 year old looking tots in TS4 would be labeled as "baby", imo. I don't want to sound rude, but what you're suggesting just sounds technically impossible.
Second, I don't see how this would help either stage. Honestly, I think the toddlers stand decently fine on their own (They're severally lacking, in terms of content, but gameplay wise they're okay). Also, the current babies are newborns, while the toddlers look 3. It seems much more sensible, to me at least, to fill in the missing gap with an overhauled baby stage rather than mashing to two into one, which sounds awkward.
Lastly, this is just a personal thing, but I just don't want a 'baby-toddler combined' lifestage. I think they should each be their own thing. Maybe if the toddlers we had were much younger, something like this could have worked, but as it stands, I think it would be much more efficient to just get a baby overhaul in the future instead of trying to combine two different very different life stages to mesh into each other.
