She can come to US as a tourist, as that's what her visa is for. Period. Nothing else possible, and you shoudn't ever let anybody know you harbor other thoughts, or they'll deny entry.
Next, she cannot work at all on a tourist visa. Even you cannot pay her for having helped with your baby. Heck, if for some reason USCIS finds out that she didn't really come for tourism, they may even consider the fact that you gave her food/shelter as payment for services. That's quite extreme, but that's how far they can stretch the concept of a job. And you'll need a $100/hr attorney to defend..
Finally, the best you can hope for is 6 months visa granted at entry for tourism/visiting family. That's the max they will stamp for B-2 entry. Extension (if at all possible) will make things unnecessarily quite complex.
Remember, if she actually comes to US, don't talk anything about baby or helping. As a B-2 visa tourist, you can do two things: TOURISM and VISIT FAMILY FOR PLEASURE. Don't say or imply you'll do anything else.
There's something called a domestic attendants visa, but given your situation, it will be highly very very unlikely (and bad idea to try) to get this.
~T
