Trouble -- On my end

Well, I guess not Trouble, but an issue that has come to my attention, and I'm balking at addressing. Basically. I've only posted once or twice before, so a little background on my story--I'm a "birthmom" who has yet to give birth ( though any day now, they keep telling me. ) We are very friendly with the adopting family, they call to check up on me, how I'm feeling, all that sort of thing. Everyone's on board, everyone's confident, and under most circumstances, it seems like so far, everything is perfect ( considering the situation, of course. ) Except one thing. After the baby is born, I've decided that she will go home with the adopting family, and stay with them in their hotel until the 72 hour deadline is up, my decision is finally made, and the papers are signed. I've also agreed that my family can see her as much as they want before that deadline, because we and the adopting family have agreed on an open adoption, and they ( the a-family ) are more then happy about all of this. ( Again, everything seems perfect so far! ) But it has come to my attention that my Mum and my grandmother are wanting to buy a carseat, so they can bring her ( the baby ) home with them sometimes, so they can have her at our house. Call me cruel, but I think this is a little too much.. and a little dangerous for their emotions. I know it's going to hurt them to see their Grandbaby/Greatgrandbaby taken away ( they've never dealt with an adoption so close to home before ), and I don't want to hurt them any more than I must. But knowing my Mom and Grama, I know that if this happens, it's going to hurt them more, they're going to get attatched, which is something I want to avoid not only for myself, but for their sakes, too. Even a little thing like a carseat is, I think, not a good thing to get, because once Baby is gone, it's just a cruel little reminder that they don't have their Grandbaby here with them. There have been other little things like this too--checking out the Baby clothes at stores when they go shopping, and even almost buying diapers. It's just a reflex, I know ( "A baby is almost here, we have to get ready!" ) and thankfully, they've stopped themselves. So far. So here's my question; what's the best way to tell them "No." I respect my Mom and Grama very much. As a child, I was taught that they are an authority I will never surpass, and you never tell you parents "No" because they know best, they've been there and dealt with it before you. But I know that at this time, I NEED to say something, but I want to do so in as gentle a way I can. Any advice out there that I should keep in mind when going about this?