How awesome for you! Congratulations! PICU is an amazing place to be for learning. I hope that the unit gets you involved and loving PICU.
As for advice, I would say start off with what you think a good peds nurse would be. Review developmental practices and just focus on being comfortable with kids. A lot of the more technical stuff you'll get in any job (central lines, meds, drips, etc.). Like any preceptorship/practicum, of course act interested. Nurses will pick up on your level of interest. Be careful of what you say, as you'll be judged on what you say and how you act quickly since you don't already have longstanding relationships with the nurses.
This is silly, but I was with a nursing student last month who spent most of the time on the cell phone or the computer working on assignments. Practicum can have slow times (duh!), but if you've got a personable preceptor, take advantage of it! Ask questions, be interested, volunteer to get your hands dirty.
Awesome! And how lucky you are to have your practicum in the PICU. I had zero ICU experience in college (mostly because I never thought I'd become a critical care nurse!) Then my very first job ended up being in the PICU. I like it a lot now, but my god, it was very tough for me at first (especially never having stepped foot in an ICU before). So see and learn as much as you can- senior practicum is what you make of it. Talk to the physicians/respiratory therapists/physical therapists, etc. Listen during rounds. Familiarize yourself with the ventilators. Watch during codes (especially pay attention to the responsibilities of the nurses). Maybe review some stuff on respiratory failure and sepsis before starting as there is a lot of that in the PICU.
You've gotten great information here. Be a sponge! Soak up everything. All PICU patients will have respiratory issues so ask for experience in performing respiratory assessments. Ask the respiratory therapist to let you practice bag-valve-mask and Jackson-Reese ventilation on their teaching dummy. (If they don't have one of those, then tape a glove over the mask and give it a go.) Practice interpreting blood gases. There are other threads here that you could mine for information and advice. Welcome to the best subspecialty in nursing!
Congratulations! I just obtained my RN license last April, so I'm a very new nurse, but I, too, did my practicum in the PICU of a regionally (nationally?) renowned Pediatric Hospital in my area. It was a great experience; not only for learning, but for practicing plenty of hands-on tasks and becoming confident with assessments.
My advice is to volunteer for any task you can (as long as you feel comfortable) and never be shy to ask what/why your preceptor may be doing a certain something. Even the small things can glean a lot of new info! Be sure to brush up on any pathophys you encounter at Practicum so that you know what's going on with your patients.
You will have a great time if you enjoy critical care and you will definitely learn a lot! Don't be shy!