If you're a nanny you'll probably need to go through a specific agency to get a visa for the US.
I recommend Imperial, Eden and Nannies Inc for overseas work. Imperial in particular were excellent at extracting me from a very difficult job. For such specialised locations it's better to just apply when you see the jobs. There are agencies I would recommend avoiding but I won't name them on a public forum. I don't mind CATing them to you but they're just my opinions.
There was someone on here seeking a nanny in the Carribean - apologies if you are in fact that person! Brain like a sieve ATM.