I'm a new grad and got offered a position in the Emergency Room. My dream job is to work in the NICU. Should I take the ER offer and then transfer to NICU?

No. You should refrain from accepting positions in either the ED or the NICU. Instead, you should seek employment on an acute medical floor for a year to gain experiences with basic medical conditions, before ever thinking of going into a specialty. NICU and ED, and other critical care settings are an absolutely absurd and dangerous place for a new grad to start off. You are imperiling lives, as well as posing a risk for legal jeopardy to the institution for whom you work, In fact, their offering you jobs in the ED or NICU as a new grad, is a real red flag for the entire institution. I would look elsewhere