What is a good strategy for getting experience in a specialty as a new grad? I am currently on a med/surg observation floor and I would love to eventually get my foot in the door for postpartum/mother baby/pediatric specialties.

I would see if the hospital you are working in has openings.
Then I would have a conversation with your current NM to share your desire to change !
As a NM I appreciate d my staff coming to me- and if I could help I did !
See if you can shadow .
Start looking in to certifications such as PALS .
Good luck

I myself went right into NICU when I graduated. I do not personally feel you need to work on a floor for a year to get that experience.
So my advice is that if there is a position in the department you want to work in, then go for it!