"Don't touch the babies!!!!"
Sorry, that was NICU! 😀
First, find out if the PICU is a closed unit (ie, the PICU is primary service or consult for the patients) or whether or not other teams admit (ie, Trauma, Peds, Peds Surg, Neurosurg, etc.).
Then, like any other rotation, make sure you know where to be, how to get there and at what time on your first day. Ask if you should "just show up" on the first day, or are expected to pre-round on patients.
Kids in the PICU will have varying problems, of varying severity and be of varying ages. You can have a hulking 16 yo right next to the 6 mo baby. Some will be intubated, some have feeding tubes, some will require frequent neuro checks, etc. Like any ICU.
There are handbooks and other Peds books, but the gold standard is the Harriet Lane handbook. Might be overkill if you aren't going into Peds, but you can get used copies on Amazon pretty cheap, or borrow it from the library. Pretty comprehensive. Also consider a PALS/Pediatric Lifesaving card for your pocket as well as a drug handbook, like Tarason's, which has Pediatric dosing.
Be able to calculate appropriate IV fluids and rates. Make sure to report urine output in cc/hr rather than by shift. Know what the kids are being fed (ie, tube feeds) or are eating. Drain(s) (if any) output...standard ICU stuff.
With regards to the first above, if there are several teams involved, it is helpful to know the plan from them - ie, if a Pediatric trauma, Ortho, Neurosurg and Peds Surg may all be involved. Check with them to see what their plans are.
Is family around? If a trauma, is non-accidental injury suspected? If so, what is social work's/CPS's input? What have the various tests and consultants said?
Oh yeah, and don't touch the babies.!