Help! My child wants a child

Last week, we spent the better part of a day driving to Cape Cod. This is pretty much what we heard for most of the ride from Sabrina:

Sabrina: "Mommy, can we have another kid?"
Me: "Daddy and I will think about it!" [As we have been for several years....]
Sabrina: "But Mommy, I want us to have a baby!"
Me: "This isn't the type of decision we can make right now!"
Sabrina: "Oh, OK."

[5 minutes later]

Sabrina: "Mommy, other families have three kids! We should too!"

Me: "Daddy and I will consider it."
Sabrina: "Mommy, don't you want another baby?"
Me: "Yes, I do, but this is a bigger decision Daddy and I have to make."
Sabrina: "OK, but can we have one?"
Me: [Pulling my hair out]

[10 minutes later]

Sabrina: "Mommy, can we have another baby?"
Me: "Sabrina, please, stop asking. This is something Daddy and I will decide."
Sabrina: "I know, but please please please can we have one? Max still needs help with stuff so you can just take care of a baby too!"

Me: "That's an interesting point. Honey, listen, I know you would like another brother or sister and maybe someday we will have one but I don't know."
Sabrina: "I really want one!"
Me: "I can tell! But this isn't something you can control, so please stop asking."
Sabrina: "Well, you can adopt!"

[About 3.5 minutes later]

Sabrina: "Mommy, I waaaaaant another baby!"

[Repeat repeat repeat repeat repeat repeat repeat]

At the restaurant we stopped at for dinner:

Me: "Sabrina, do you want a salad with grilled chicken?"
Sabrina: "You KNOW what I want."
Me: "Yes, but what do you want for dinner?"
Sabrina: "A baby."

[5 minutes later]

Sabrina: "How do you get pregnant?"
Me: "Like we've talked about: The Mommy and Daddy love each other and make a baby."
Sabrina: "But really what happens?"

Me: We've talked about that too! The Daddy's penis lets out a seed inside the Mommy's vagina."
Dave: "HONEY!!! She is too young for that!"
Me: "Um, no, she's not."

Sabrina: "Ewwww, I can't believe you said that at dinner!"

That actually succeeded in clamming her up.

[Back at home several days later]

Sabrina (calling from the mall): "Mommy! Can I get a dog? They are doing free dog adoptions here!"
Me: "No, honey, I'm sorry, we have to think that over."
Sabrina: "OK. Then can we get a baby?"
Me: [Pulls out rest of hair.]