Sadly, it sounds like you're going to have to be blatently rude about it. If/When they come over, flat out refuse.
I don't disagree with most of what Laura_Elaine said, but I totally disagree with the idea that you have to be rude. Refusing the let them take your newborn away from you doesn't have to be intrinsically rude. You can blatantly turn them down in a polite way. Sometimes I think as women, we're trained to think that no=rude, but it really doesn't.
"We're going to take baby next week for you. We're so helpful!"
"That won't be possible. It's really important for me and baby to bond during these early months. Maybe when she's 10-12 months old [OP, insert whatever time you think is appropriate] we'll be ready for her to spend a few days away from home."
"You're so ungrateful! We're trying to help!"
"I really appreciate the impulse! You know what'd be really helpful, though? If you could spare the time, I could really use some help with the [laundry/vacuuming/dusting/whatever you want to put in here]. All of my time right now is being spent taking care of baby and getting to know him/her, and the house is such a mess! I totally understand if you can't help out in that way, though."
If you just want them to leave you don't have to go with the "help with housework" bit, of course. You could just say "We're really not ready for visitors yet. When we are, I'll give you a call and we can plan a visit!" or something.
Whatever you do, decide what you're comfortable with, set boundaries, and remain firm. They may give you a hard time, but know that you're doing what's best for your LO and your family. If you keep firm boundaries, they'll eventually have to accept them. They're not going to take your baby if you say no - that'd be kidnapping! Plus, you'll be setting the stage for the future. If you can stand your ground here, next time they want to pressure you into something, they'll likely back down more quickly because they know that doesn't work well on you.