Isn't that frustrating? You spend lots of time knitting a beautiful gift for someone and they don't use it.
It's a risk we take as knitters and I am in the school of thought that says once you gift it, it's no longer yours.
The bear/doll idea is very cute, but it's not really your place to decide what they do with it. Unfortunately it's not yours. I really really do understand what you're going through, but it's a gift.
What if you'd bought them a diaper bag and they never used it and you knew they didn't. Would you ask for it back? Most likely not.
I would probably ask if they had ever used it and when they say no, mention that it is for your neice and you really wanted it to be worn. I like the idea that someone else gave about letting the mother pick out a pattern she likes for the next outfit.