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.

Start studying for your certification in those areas you want to work and then talk yourself up to the people who manage those areas and ask them about your opportunities for growth. They will love your interest and this will help you get a foot in.

Some hospitals offer "crossover" or "new to specialty" training. If you have OB in your hospital, let the manager know that you wish to transfer. Otherwise, find a hospital that has new to specialty training.

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!