Long player here.
I would LOVE for babies to be detached from their prison-bassinet. There was a playmat and a highchair that baby could be placed in that allowed for the infant to play, available in the Sims 2 Store. The highchair/play chair was good for the attending Sim to be able to fix a meal. Babies do need to be bathed. It was fun to watch a Sim hold the newly fed infant above their head and for baby to spit-up all over said Sim. Bathing them in the sink was also fun, plus it promoted a growth in relationship between the attending Sim and the baby.
If another life stage is considered, it should be the Infant stage, where a baby learns how to sit up and crawl. Also, they should have a walker to place them in wherein the baby can scoot around the room, too. If not included, we don't need such a life-stage. What we have now is a never-ending newborn stage.
Babies should be babies, again and not an object as they are now. They may have several interactions, but they aren't a smooth transition. You shouldn't have to place the baby back into the bassinet, just to remove them again for the next interaction. Face/palm. Please untether the child.
